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As filed with the Securities and Exchange Commission on November 21, 2024

Registration No. 333-

UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549

FORM S-4

REGISTRATION STATEMENT
UNDER
THE SECURITIES ACT OF 1933

ATLANTIC UNION BANKSHARES CORPORATION
(Exact Name of Registrant as Specified in Its Charter)

Virginia

(State or Other Jurisdiction of
Incorporation or Organization)

6022

(Primary Standard Industrial
Classification Code Number)

54-1598552

(I.R.S. Employer
Identification Number)

4300 Cox Road
Glen Allen, Virginia 23060
(804) 633-5031

(Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices)

John C. Asbury
President and Chief Executive Officer
Atlantic Union Bankshares Corporation
4300 Cox Road
Glen Allen, Virginia 23060
(804) 633-5031

(Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent For Service)

Copies to:

Lee Hochbaum, Esq.
Davis Polk & Wardwell LLP
450 Lexington Avenue
New York, New York 10017
(212) 450-4000

Daniel J. Schrider
Chair, President and Chief Executive Officer
Sandy Spring Bancorp, Inc.
17801 Georgia Avenue
Olney, Maryland 20832
(301) 774-6400

Edward G. Olifer, Esq.
Stephen F. Donahoe, Esq.
Kilpatrick Townsend & Stockton LLP
701 Pennsylvania Avenue NW, Suite 200
Washington, DC 20004
(202) 508-5800

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this Registration Statement is declared effective and upon completion of the merger described herein.

If the securities being registered on this Form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box: ☐

If this Form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

If this Form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company,” and “emerging growth company” in Rule 12b-2 of the Exchange Act.

	Large accelerated filer ☒			Accelerated filer ☐
	Non-accelerated filer ☐			Smaller reporting company ☐
				Emerging growth company ☐

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of the Securities Act. ☐

If applicable, place an X in the box to designate the appropriate rule provision relied upon in conducting this transaction:

Exchange Act Rule 13e-4(i) (Cross-Border Issuer Tender Offer) ☐

Exchange Act Rule 14d-l(d) (Cross-Border Third-Party Tender Offer) ☐

THE REGISTRANT HEREBY AMENDS THIS REGISTRATION STATEMENT ON SUCH DATE OR DATES AS MAY BE NECESSARY TO DELAY ITS EFFECTIVE DATE UNTIL THE REGISTRANT SHALL FILE A FURTHER AMENDMENT WHICH SPECIFICALLY STATES THAT THIS REGISTRATION STATEMENT SHALL THEREAFTER BECOME EFFECTIVE IN ACCORDANCE WITH SECTION 8(A) OF THE SECURITIES ACT OF 1933 OR UNTIL THE REGISTRATION STATEMENT SHALL BECOME EFFECTIVE ON SUCH DATE AS THE SECURITIES AND EXCHANGE COMMISSION, ACTING PURSUANT TO SAID SECTION 8(A), MAY DETERMINE.

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The information in this joint proxy statement/prospectus is not complete and may be changed. A registration statement relating to the securities described in this joint proxy statement/prospectus has been filed with the U.S. Securities and Exchange Commission. These securities may not be issued until the registration statement filed with the U.S. Securities and Exchange Commission is effective. This joint proxy statement/prospectus does not constitute an offer to sell or the solicitation of offers to buy these securities in any jurisdiction where the offer or sale is not permitted.

PRELIMINARY — SUBJECT TO COMPLETION — DATED NOVEMBER 21, 2024

To the Shareholders of Atlantic Union Bankshares Corporation
and the Stockholders of Sandy Spring Bancorp, Inc.
MERGER PROPOSED — YOUR VOTE IS VERY IMPORTANT

On behalf of the boards of directors of Atlantic Union Bankshares Corporation (“Atlantic Union”) and Sandy Spring Bancorp, Inc. (“Sandy Spring”), we are pleased to enclose the accompanying joint proxy statement/prospectus relating to the proposed acquisition of Sandy Spring by Atlantic Union. We are requesting that you take certain actions as a holder of Atlantic Union common stock or a holder of Sandy Spring common stock.

The boards of directors of Atlantic Union and Sandy Spring have each unanimously approved an agreement for Atlantic Union to acquire Sandy Spring. Pursuant to the Agreement and Plan of Merger, dated as of October 21, 2024, by and between Atlantic Union and Sandy Spring (as amended from time to time, the “merger agreement”), Sandy Spring will merge with and into Atlantic Union (the “merger”), with Atlantic Union surviving the merger (the “surviving corporation” or “Atlantic Union,” as the case may be).

The proposed merger will leverage the strengths of both organizations, creating the largest regional bank headquartered in the lower Mid-Atlantic, and significantly enhancing the combined company’s presence in Northern Virginia and Maryland. We believe the merger will position the surviving corporation as a premier banking institution, with increased scale, an expanded geographic footprint and an enhanced platform for future growth. In addition, we believe that the surviving corporation will benefit from the talented management teams, similar core values and strong commitments to serving their customers and communities of both Atlantic Union and Sandy Spring.

In the merger, holders of Sandy Spring common stock will receive 0.900 shares (the “exchange ratio” and such shares, the “merger consideration”) of Atlantic Union common stock for each share of Sandy Spring common stock they own. Holders of Atlantic Union common stock will continue to own their existing shares of Atlantic Union common stock. Based on the closing price of Atlantic Union common stock on the New York Stock Exchange (the “NYSE”) on October 18, 2024, the last trading day before public announcement of the merger, the exchange ratio represented approximately $34.929 in value for each share of Sandy Spring common stock. Based on the closing price of Atlantic Union common stock on the NYSE on [ ], the last practicable trading day before the date of the accompanying joint proxy statement/prospectus, of $[ ], the exchange ratio represented approximately $[ ] in value for each share of Sandy Spring common stock. The value of Sandy Spring common stock at the closing date of the merger (the “closing date”) could be greater than, less than or the same as the value of Atlantic Union common stock on the date of the accompanying joint proxy statement/prospectus. We urge you to obtain current market quotations of Atlantic Union common stock (trading symbol “AUB”) and Sandy Spring common stock (trading symbol “SASR”).

We expect that the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended. Accordingly, holders of Sandy Spring common stock generally will not recognize any gain or loss for U.S. federal income tax purposes on the exchange of shares of Sandy Spring common stock for Atlantic Union common stock in the merger, except with respect to any cash received instead of fractional shares of Atlantic Union common stock.

Based on the number of shares of Sandy Spring common stock outstanding and reserved for issuance as of [ ], 2024, Atlantic Union expects to issue approximately [ ] million shares of Atlantic Union common stock in the merger. Following the completion of the merger, former holders of Sandy Spring common stock will own approximately 29% and existing holders of Atlantic Union common stock will own approximately 71% of the common stock of the surviving corporation.

The special meeting of holders of Atlantic Union common stock will be held on [ ] virtually via the Internet, at [ ], at [ ] a.m. Eastern Time. The special meeting of holders of Sandy Spring common stock will be held on [ ] at [ ], at [ ] a.m. Eastern Time. At our respective special meetings, in addition to other business, Atlantic Union will ask its shareholders to approve the merger agreement and the transactions contemplated thereby, including the merger and the issuance of shares of Atlantic Union common stock to holders of Sandy Spring common stock pursuant to the merger agreement (including for purposes of complying with NYSE Listing Rule 312.03, which requires approval of the issuance of shares of Atlantic Union common stock in an amount that exceeds 20% of the currently outstanding shares of Atlantic Union common stock) (the “Atlantic Union merger proposal”), and Sandy Spring will ask its

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stockholders to approve the merger agreement and the transactions contemplated thereby (the “Sandy Spring merger proposal”). Information about these meetings and the merger is contained in the accompanying joint proxy statement/prospectus. In particular, see the section entitled “Risk Factors” beginning on page 26. We urge you to read the accompanying joint proxy statement/prospectus carefully and in its entirety.

Holders of Atlantic Union preferred stock and holders of depositary shares representing a 1/400th interest in a share of Atlantic Union preferred stock are not entitled to, and are not requested to, vote at the Atlantic Union special meeting.

Whether or not you plan to attend your special meeting, please vote as soon as possible to make sure that your shares are represented at the meeting. If you do not vote, it will have the same effect as voting “AGAINST” the Atlantic Union merger proposal and the Sandy Spring merger proposal, as applicable.

Each of our boards of directors unanimously recommends that holders of common stock vote “FOR” each of the proposals to be considered at the respective meetings. We strongly support this combination of our companies and join our boards of directors in their recommendations.

John C. Asbury
President and Chief Executive Officer
Atlantic Union Bankshares Corporation

Daniel J. Schrider
Chair, President and Chief Executive Officer
Sandy Spring Bancorp, Inc.

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the securities to be issued in connection with the merger or determined if this document is accurate or complete. Any representation to the contrary is a criminal offense.

The securities to be issued in the merger are not savings or deposit accounts or other obligations of any bank or non-bank subsidiary of either Atlantic Union or Sandy Spring, and they are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

The accompanying joint proxy statement/prospectus is dated [ ], and is first being mailed to holders of Atlantic Union common stock and holders of Sandy Spring common stock on or about [ ].

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Atlantic Union Bankshares Corporation
4300 Cox Road
Glen Allen, Virginia 23060
Notice of Special Meeting of Shareholders

To the Shareholders of Atlantic Union Bankshares Corporation:

On October 21, 2024, Atlantic Union Bankshares Corporation (“Atlantic Union”) and Sandy Spring Bancorp, Inc. (“Sandy Spring”) entered into an Agreement and Plan of Merger (as amended from time to time, the “merger agreement”), a copy of which is attached as Annex A to the accompanying joint proxy statement/prospectus.

NOTICE IS HEREBY GIVEN that a special meeting of holders of common stock, par value $1.33 per share, of Atlantic Union (“Atlantic Union common stock” and such meeting, the “Atlantic Union special meeting”) will be held on [ ] virtually via the Internet, at [ ] (the “Atlantic Union special meeting website”), at [ ] a.m. Eastern Time. We are pleased to notify you of and invite you to the Atlantic Union special meeting.

At the Atlantic Union special meeting, you will be asked to consider and vote on the following matters:

•

Proposal to approve the merger agreement and the transactions contemplated thereby, including the merger and the issuance of shares of Atlantic Union common stock to holders of Sandy Spring common stock pursuant to the merger agreement (including for purposes of complying with NYSE Listing Rule 312.03, which requires approval of the issuance of shares of Atlantic Union common stock in an amount that exceeds 20% of the currently outstanding shares of Atlantic Union common stock) (the “Atlantic Union merger proposal”); and

•

Proposal to adjourn or postpone the Atlantic Union special meeting, if necessary or appropriate, to solicit additional proxies if, immediately prior to such adjournment or postponement, there are not sufficient votes to approve the Atlantic Union merger proposal or to ensure that any supplement or amendment to the accompanying joint proxy statement/prospectus is timely provided to holders of Atlantic Union common stock (the “Atlantic Union adjournment proposal”).

The Atlantic Union board of directors has fixed the close of business on [ ] as the record date for the Atlantic Union special meeting. Only holders of record of Atlantic Union common stock as of the close of business on the record date for the Atlantic Union special meeting are entitled to notice of, and to vote at, the Atlantic Union special meeting or any adjournment or postponement thereof.

The Atlantic Union board of directors unanimously recommends that holders of Atlantic Union common stock vote “FOR” the Atlantic Union merger proposal and “FOR” the Atlantic Union adjournment proposal.

Atlantic Union has determined that holders of Atlantic Union common stock are not entitled to appraisal or dissenters’ rights with respect to the merger contemplated under the merger agreement under Section 13.1-730 of the of the Virginia Stock Corporation Act (“Virginia law”).

Your vote is important. We cannot complete the transactions contemplated by the merger agreement unless holders of Atlantic Union common stock approve the Atlantic Union merger proposal. The affirmative vote of a majority of all the votes entitled to be cast at the Atlantic Union special meeting is required to approve the Atlantic Union merger proposal.

Each copy of the joint proxy statement/prospectus mailed to holders of Atlantic Union common stock is accompanied by a form of proxy card with instructions for voting.

Whether or not you plan to attend the Atlantic Union special meeting, we urge you to please promptly complete, sign, date and return the accompanying proxy card in the enclosed postage-paid envelope or authorize the individuals named on the accompanying proxy card to vote your shares by calling the toll-free

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telephone number or by using the Internet as described in the instructions included with the accompanying proxy card. If your shares are held in the name of a bank, broker, trustee or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker, trustee or other nominee.

The joint proxy statement/prospectus of which this notice is part provides a detailed description of the merger agreement, the merger contemplated thereby and the other matters to be considered at the Atlantic Union special meeting. A summary of the merger agreement is included in the joint proxy statement/prospectus in the sections entitled “The Merger” and “The Transaction Agreements — Description of the Merger Agreement,” and a copy of the merger agreement is attached as Annex A to the joint/proxy statement prospectus, each of which are incorporated by reference into this notice to the same extent as if fully set forth herein. We encourage you to carefully read this joint proxy statement/prospectus (including the annexes thereto) and any other documents incorporated by reference herein in their entirety.

If you have any questions regarding the accompanying joint proxy statement/prospectus, you may contact [ ], Atlantic Union’s proxy solicitor, by calling toll-free at ([ ]) [ ]-[ ], or for banks and brokers, collect at ([ ]) [ ]-[ ].

By Order of the Board of Directors

Rachael R. Lape
Executive Vice President,
General Counsel and Corporate Secretary
Atlantic Union Bankshares Corporation

[ ]

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Sandy Spring Bancorp, Inc.
17801 Georgia Avenue
Olney, Maryland 20832

Notice of Special Meeting of Stockholders

To the Stockholders of Sandy Spring Bancorp, Inc.:

On October 21, 2024, Sandy Spring Bancorp, Inc. (“Sandy Spring”) and Atlantic Union Bankshares Corporation (“Atlantic Union”) entered into an Agreement and Plan of Merger (as amended from time to time, the “merger agreement”), a copy of which is attached as Annex A to the accompanying joint proxy statement/prospectus.

NOTICE IS HEREBY GIVEN that a special meeting of holders of common stock, par value $1.00 per share, of Sandy Spring (“Sandy Spring common stock” and such meeting, the “Sandy Spring special meeting”) will be held on [ ], at [ ] .m. Eastern Time. We are pleased to notify you of and invite you to the Sandy Spring special meeting.

At the Sandy Spring special meeting, you will be asked to consider and vote on the following matters:

•

Proposal to approve the merger agreement and the transactions contemplated thereby (the “Sandy Spring merger proposal”);

•

Proposal to approve, on a non-binding, advisory basis, the compensation that may be paid or become payable to Sandy Spring’s named executive officers that is based on or otherwise relates to the merger (the “Sandy Spring compensation proposal”); and

•

Proposal to adjourn or postpone the Sandy Spring special meeting, if necessary or appropriate, to solicit additional proxies if, immediately prior to such adjournment or postponement, there are not sufficient votes to approve the Sandy Spring merger proposal or to ensure that any supplement or amendment to the accompanying joint proxy statement/prospectus is timely provided to holders of Sandy Spring common stock (the “Sandy Spring adjournment proposal”).

The Sandy Spring board of directors has fixed the close of business on [ ] as the record date for the Sandy Spring special meeting. Only holders of record of Sandy Spring common stock as of the close of business on the record date for the Sandy Spring special meeting are entitled to notice of, and to vote at, the Sandy Spring special meeting or any adjournment or postponement thereof.

The Sandy Spring board of directors unanimously recommends that holders of Sandy Spring common stock vote “FOR” the Sandy Spring merger proposal, “FOR” the Sandy Spring compensation proposal, and “FOR” the Sandy Spring adjournment proposal.

Sandy Spring has determined that holders of Sandy Spring common stock are not entitled to appraisal or dissenters’ rights with respect to the merger contemplated under the merger agreement under Section 3-202 of the Maryland General Corporation Law (“Maryland law”).

Your vote is important. We cannot complete the transactions contemplated by the merger agreement unless the holders of Sandy Spring common stock approve the Sandy Spring merger proposal. The affirmative vote of 66 2/3% of all the votes entitled to be cast by holders of Sandy Spring common stock is required to approve the Sandy Spring merger proposal.

Each copy of the joint proxy statement/prospectus mailed to holders of Sandy Spring common stock is accompanied by a form of proxy card with instructions for voting.

Whether or not you plan to attend the Sandy Spring special meeting, we urge you to please promptly complete, sign, date and return the accompanying proxy card in the enclosed postage-paid envelope or

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authorize the individuals named on the accompanying proxy card to vote your shares by calling the toll-free telephone number or by using the Internet as described in the instructions included with the accompanying proxy card. If your shares are held in the name of a bank, broker, trustee or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker, trustee or other nominee.

The joint proxy statement/prospectus of which this notice is part provides a detailed description of the merger agreement, the merger contemplated thereby and the other matters to be considered at the Sandy Spring special meeting. A summary of the merger agreement is included in the joint proxy statement/prospectus in the sections entitled “The Merger” and “The Transaction Agreements — Description of the Merger Agreement,” and a copy of the merger agreement is attached as Annex A to the joint/proxy statement prospectus, each of which are incorporated by reference into this notice to the same extent as if fully set forth herein. We encourage you to carefully read this joint proxy statement/prospectus (including the annexes thereto) and any other documents incorporated by reference herein in their entirety.

If you have any questions regarding the accompanying joint proxy statement/prospectus, you may contact [ ], Sandy Spring’s proxy solicitor, by calling toll-free at ([ ]) [ ]-[ ], or for banks and brokers, collect at ([ ]) [ ]- [ ], or by email at [ ].

By Order of the Board of Directors

Aaron M. Kaslow
General Counsel, Chief Administrative Officer & Secretary

[ ]

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ADDITIONAL INFORMATION

As permitted by the rules of the Securities and Exchange Commission (the “SEC”), this joint proxy statement/prospectus incorporates important business and financial information about Atlantic Union and Sandy Spring from other documents that are not included in or delivered with this joint proxy statement/prospectus. This information is available to you without charge upon your written or oral request. You can obtain the documents incorporated by reference in this document through the SEC website at http://www.sec.gov.

Copies of documents filed by Atlantic Union with the SEC are available at the investor relations page of Atlantic Union’s website, https://investors.atlanticunionbank.com, and are also available to you free of charge upon your request in writing or by telephone to Atlantic Union at the address and telephone number below. Copies of documents filed by Sandy Spring with the SEC are available at the investor relations page of Sandy Spring’s website, https://sandyspringbancorp.q4ir.com/overview/default.aspx, and are also available to you free of charge upon your request in writing or by telephone to Sandy Spring at the address and telephone number below.

	• if you are an Atlantic Union shareholder:			• if you are a Sandy Spring stockholder:
	Atlantic Union Bankshares Corporation 4300 Cox Road Glen Allen, Virginia 23060 (804) 633-5031			Sandy Spring Bancorp, Inc. 17801 Georgia Avenue Olney, Maryland 20832 (301) 774-6400
	Attention: Rachael R. Lape Executive Vice President, General Counsel and Corporate Secretary			Attention: Investor Relations

You will not be charged for any of these documents that you request. To obtain timely delivery of these documents, you must make your request no later than five business days before the date of your applicable special meeting. This means that holders of Atlantic Union common stock requesting documents must do so by [ ], in order to receive them before the Atlantic Union special meeting, and holders of Sandy Spring common stock requesting documents must do so by [ ] in order to receive them before the Sandy Spring special meeting.

See the section of the accompanying joint proxy statement/prospectus entitled “Where You Can Find More Information” for further information. The contents of the websites of the SEC, Atlantic Union and Sandy Spring are not being incorporated into this proxy statement/prospectus. This information about how you can obtain certain documents that are being incorporated by reference into this joint proxy statement/prospectus at these websites is being provided only for your convenience.

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ABOUT THIS JOINT PROXY STATEMENT/PROSPECTUS

This joint proxy statement/prospectus, which forms part of a registration statement on Form S-4 filed with the SEC by Atlantic Union, constitutes a prospectus of Atlantic Union under Section 5 of the Securities Act of 1933, as amended (the “Securities Act”), with respect to the shares of Atlantic Union common stock to be issued to holders of Sandy Spring common stock pursuant to the merger agreement. This joint proxy statement/prospectus also constitutes a proxy statement of each of Atlantic Union and Sandy Spring under Section 14(a) the Securities Exchange Act of 1934, as amended (the “Exchange Act”). This joint proxy statement/prospectus also constitutes a notice of meeting with respect to the Atlantic Union special meeting and a notice of meeting with respect to the Sandy Spring special meeting.

Except where the context otherwise indicates, information contained in, or incorporated by reference into, this document regarding Sandy Spring has been provided by Sandy Spring and information contained in, or incorporated by reference into, this document regarding Atlantic Union has been provided by Atlantic Union.

You should rely only on the information contained in, or incorporated by reference into, this joint proxy statement/prospectus. No one has been authorized to provide you with information that is different from that contained in, or incorporated by reference into, this joint proxy statement/prospectus. This joint proxy statement/prospectus is dated [ ], and you should assume that the information in this document is accurate only as of such date. You should assume that the information incorporated by reference into this document is accurate as of the date of such incorporated document. Neither the mailing of this joint proxy statement/prospectus to holders of Atlantic Union common stock or holders of Sandy Spring common stock nor the issuance by Atlantic Union of shares of Atlantic Union common stock in connection with the merger will create any implication to the contrary.

This joint proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction.

All currency amounts referenced in this proxy statement/prospectus are in U.S. dollars. In this joint proxy statement/prospectus, except as otherwise indicated or the context otherwise requires, references to:

•

“Atlantic Union” refers to Atlantic Union Bankshares Corporation, a Virginia corporation;

•

“Atlantic Union articles of incorporation” refers to the Amended and Restated Articles of Incorporation of Atlantic Union, effective May 7, 2020;

•

“Atlantic Union Bank” refers to Atlantic Union Bank, a Federal Reserve member bank chartered under the laws of the Commonwealth of Virginia and a wholly owned subsidiary of Atlantic Union;

•

“Atlantic Union bylaws” refers to the Amended and Restated Bylaws of Atlantic Union, effective as of December 6, 2023;

•

“Atlantic Union board of directors” refers to the board of directors of Atlantic Union;

•

“Atlantic Union common stock” refers to the common stock, par value $1.33 per share, of Atlantic Union;

•

“Atlantic Union support agreement” refers to the Support Agreement, by and between Sandy Spring Bancorp, Inc. and each of the shareholders of Atlantic Union Bankshares Corporation listed on the signature pages therein, dated as of October 21, 2024;

•

“closing” refers to the closing of the merger;

•

“closing date” refers to the date on which the closing occurs;

•

“forward sale agreements” refers to the forward sale agreements between Atlantic Union and Morgan Stanley & Co. LLC, as forward purchaser, each dated as of October 21, 2024, in which the forward purchaser or its affiliate borrowed from third parties an aggregate of 11,338,028 shares of Atlantic Union common stock;

•

“KBW” refers to Keefe, Bruyette & Woods, Inc., financial advisor to Sandy Spring;

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•

“Maryland law” or the “MGCL” refers to the Maryland General Corporation Law, as amended;

•

“merger” refers to the merger of Sandy Spring with and into Atlantic Union, with Atlantic Union surviving as the surviving corporation.

•

“Morgan Stanley” refers to Morgan Stanley & Co. LLC, financial advisor to Atlantic Union;

•

“Sandy Spring” refers to Sandy Spring Bancorp, Inc., a Maryland corporation;

•

“Sandy Spring Bank” refers to Sandy Spring Bank, a Federal Reserve member bank chartered under the laws of the State of Maryland and a wholly owned subsidiary of Sandy Spring;

•

“Sandy Spring board of directors” refers to the board of directors of Sandy Spring;

•

“Sandy Spring articles of incorporation” refers to the Amended and Restated Articles of Incorporation of Sandy Spring, dated as of January 22, 1992, as amended;

•

“Sandy Spring bylaws” refers to the Amended and Restated Bylaws of Sandy Spring, dated as of December 14, 2022;

•

“Sandy Spring common stock” refers to the common stock, par value $1.00 per share, of Sandy Spring;

•

“Sandy Spring support agreement” refers to the Support Agreement, by and between Atlantic Union Bankshares Corporation and each of the stockholders of Sandy Spring Bancorp, Inc. listed on the signature pages therein, dated as of October 21, 2024;

•

“SEC” refers to the U.S. Securities and Exchange Commission;

•

“surviving corporation” refers to Atlantic Union after Sandy Spring merges into Atlantic Union as part of the merger, with Atlantic Union surviving the merger;

•

“transactions” refers to the merger and the other transactions contemplated by the merger agreement; and

•

“Virginia law” or the “VSCA” refers to the Virginia Stock Corporation Act, as amended.

iii

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QUESTIONS AND ANSWERS

Why am I receiving this joint proxy statement/prospectus?

You are receiving this joint proxy statement/prospectus because Sandy Spring and Atlantic Union have entered into an Agreement and Plan of Merger, dated as of October 21, 2024 (as it may be amended from time to time, the “merger agreement”). A copy of the merger agreement is attached as Annex A to this joint proxy statement/prospectus and is incorporated by reference herein. Under the merger agreement, subject to the satisfaction (or to the extent permitted by law and in accordance with the merger agreement, waiver) of the conditions to the merger set forth in the merger agreement, Sandy Spring will merge with and into Atlantic Union, so that Atlantic Union is the surviving corporation (sometimes referred to in such capacity as the “surviving corporation”). Following the completion of the merger, Sandy Spring Bank will merge with and into Atlantic Union Bank (the “bank merger” and together with the merger, the “transactions”), with Atlantic Union Bank as the surviving bank (the “surviving bank”).

To complete the merger, among other things:

•

holders of Sandy Spring common stock must approve the merger agreement and the transactions contemplated thereby (the “Sandy Spring merger proposal”); and

•

holders of Atlantic Union common stock must approve the merger agreement and the transactions contemplated thereby, including the merger and the issuance of shares of Atlantic Union common stock to holders of Sandy Spring common stock pursuant to the merger agreement (including for purposes of complying with NYSE Listing Rule 312.03, which requires approval of the issuance of shares of Atlantic Union common stock in an amount that exceeds 20% of the currently outstanding shares of Atlantic Union common stock) (the “Atlantic Union share issuance” and such proposal the “Atlantic Union merger proposal”).

Atlantic Union is holding a special meeting of holders of Atlantic Union common stock (the “Atlantic Union special meeting”) to obtain approval of the Atlantic Union merger proposal. Holders of Atlantic Union common stock will also be asked to approve the proposal to adjourn or postpone the Atlantic Union special meeting, if necessary or appropriate, to solicit additional proxies if, immediately prior to such adjournment or postponement, there are not sufficient votes at the time of the Atlantic Union special meeting to approve the Atlantic Union merger proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to the holders of Atlantic Union common stock (the “Atlantic Union adjournment proposal”).

Concurrently with the execution and delivery of the merger agreement, each of the members of the board of directors of Atlantic Union entered into a support agreement pursuant to which, among other things, each of the members of the board of directors of Atlantic Union agreed, subject to the terms of the support agreement, to (i) vote the shares of Atlantic Union common stock over which he or she has the sole power to vote or direct the voting of in favor of the approval of the merger agreement and the Atlantic Union share issuance, and against any competing transaction and (ii) not transfer any such shares of Atlantic Union common stock prior to the Atlantic Union special meeting, with certain limited exceptions. The support agreements will terminate upon the earlier of the termination of the merger agreement or the effective time. As of [ ], the record date for the Atlantic Union special meeting, the members of the board of directors of Atlantic Union owned and held the sole dispositive and voting power over shares of Atlantic Union common stock representing approximately [ ]% of the voting power represented by all issued and outstanding shares of Atlantic Union common stock. A copy of the support agreement is attached to this joint proxy statement/prospectus as Annex B.

Sandy Spring is holding a special meeting of holders of Sandy Spring common stock (the “Sandy Spring special meeting”) to obtain approval of the Sandy Spring merger proposal. Holders of Sandy Spring common stock will also be asked to (i) approve, on a non-binding, advisory basis, the compensation that may be paid or become payable to Sandy Spring’s named executive officers that is based on or otherwise relates to the merger (the “Sandy Spring compensation proposal”) and (ii) approve

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the proposal to adjourn or postpone the Sandy Spring special meeting, if necessary or appropriate, to solicit additional proxies if, immediately prior to such adjournment or postponement, there are not sufficient votes at the time of the Sandy Spring special meeting to approve the Sandy Spring merger proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to holders of Sandy Spring common stock (the “Sandy Spring adjournment proposal”).

Concurrently with the execution and delivery of the merger agreement, each of the members of the board of directors of Sandy Spring entered into a support agreement pursuant to which, among other things, each of the members of the board of directors of Sandy Spring agreed, subject to the terms of the support agreement, to (i) vote the shares of Sandy Spring common stock over which he or she has the sole power to vote or direct the voting of in favor of the approval of the merger proposal, and against any competing transaction and (ii) not transfer any such shares of Sandy Spring common stock prior to the Sandy Spring special meeting, with certain limited exceptions. The support agreements will terminate upon the earlier of the termination of the merger agreement or the effective time. As of [ ], the record date for the Sandy Spring special meeting, the members of the board of directors of Sandy Spring owned and held the sole dispositive and voting power over shares of Sandy Spring common stock representing approximately [ ]% of the voting power represented by all issued and outstanding shares of Sandy Spring common stock. A copy of the support agreement is attached to this joint proxy statement/prospectus as Annex C.

Neither the holders of Sandy Spring common stock nor the holders of Atlantic Union common stock are entitled to appraisal or dissenters’ rights in connection with the merger.

This document is also a prospectus that is being delivered to holders of Sandy Spring common stock because, in connection with the merger, Atlantic Union is offering shares of Atlantic Union common stock to holders of Sandy Spring common stock.

This joint proxy statement/prospectus contains important information about the merger and the proposals being voted on at the Atlantic Union special meeting and the Sandy Spring special meeting. You should read it carefully and in its entirety. The enclosed materials allow you to have your shares of Atlantic Union common stock or Sandy Spring common stock, as applicable, voted by proxy without attending your respective meeting. Your vote is important and we encourage you to submit your proxy as soon as possible.

What will happen in the merger?

In the merger, Sandy Spring will merge with and into Atlantic Union, so that Atlantic Union is the surviving corporation. Each share of Sandy Spring common stock issued and outstanding immediately prior to the effective time of the merger (the “effective time”), other than certain shares held by Atlantic Union or Sandy Spring, will be converted into the right to receive 0.900 shares (the “exchange ratio”) of Atlantic Union common stock (and cash in lieu of fractional shares) (the “merger consideration”).

After completion of the merger, Sandy Spring will cease to exist and will no longer be a public company and Sandy Spring common stock will be delisted from the Nasdaq Global Select Market (“Nasdaq”), deregistered under the Exchange Act and cease to be publicly traded. Holders of Atlantic Union common stock will continue to own their existing shares of Atlantic Union common stock. See the information provided in the section entitled “The Transaction Agreements — Description of the Merger Agreement — Structure of the Merger” and the merger agreement attached as Annex A to this joint proxy statement/prospectus for more information about the merger.

When and where will each of the special meetings take place?

The Atlantic Union special meeting will be held virtually via the Atlantic Union special meeting website, which can be accessed at [ ], on [ ] at [ ] .m. Eastern Time.

The Sandy Spring special meeting will be held on [ ] at [ ] .m. Eastern Time.

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described below so that your vote will be counted even if you later decide not to or become unable to attend the applicable special meeting.

What matters will be considered at each of the special meetings?

At the Atlantic Union special meeting, holders of Atlantic Union common stock will be asked to consider and vote on the following proposals:

Proposal 1: Atlantic Union merger proposal: approval and adoption of the merger agreement and the transactions contemplated thereby, including the merger and the issuance of the shares of Atlantic Union common stock constituting the merger consideration pursuant to the merger agreement;

Proposal 2: Atlantic Union adjournment proposal: approval of the adjournment or postponement of the Atlantic Union special meeting, if necessary or appropriate, to solicit additional proxies if, immediately prior to such adjournment or postponement, there are not sufficient votes at the time of the Atlantic Union special meeting to approve the Atlantic Union merger proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to the holders of Atlantic Union common stock.

At the Sandy Spring special meeting, holders of Sandy Spring common stock will be asked to consider and vote on the following proposals:

Proposal 1: Sandy Spring merger proposal: approval and adoption of the merger agreement and the transactions contemplated thereby;

Proposal 2: Sandy Spring compensation proposal: approval, on a non-binding, advisory basis, of the compensation that may be paid or become payable to Sandy Spring’s named executive officers that is based on or otherwise relates to the merger; and

Proposal 3: Sandy Spring adjournment proposal: approval of the adjournment or postponement of the Sandy Spring special meeting, if necessary or appropriate, to solicit additional proxies if, immediately prior to such adjournment or postponement, there are not sufficient votes at the time of the Sandy Spring special meeting to approve the Sandy Spring merger proposal or to ensure that any supplement or amendment to this joint proxy statement/prospectus is timely provided to holders of Sandy Spring common stock.

In order to complete the merger, among other things, holders of Atlantic Union common stock must approve the Atlantic Union merger proposal and holders of Sandy Spring common stock must approve the Sandy Spring merger proposal. None of the approvals of the Atlantic Union adjournment proposal, the Sandy Spring compensation proposal or the Sandy Spring adjournment proposal are conditions to the obligations of Atlantic Union or Sandy Spring to complete the merger.

What will holders of Sandy Spring common stock receive at the effective time?

At the effective time, holders of Sandy Spring common stock will receive 0.900 shares of Atlantic Union common stock for each share of Sandy Spring common stock held immediately prior to the effective time (other than certain shares held by Atlantic Union or Sandy Spring). Atlantic Union will not issue any fractional shares of Atlantic Union common stock at the effective time. Holders of Sandy Spring common stock who would otherwise be entitled to a fractional share of Atlantic Union common stock will instead receive an amount in cash (rounded to the nearest cent) determined by multiplying the average of the closing-sale prices per share of Atlantic Union common stock on the NYSE for the consecutive period of five full trading days immediately preceding (but not including) the day on which the merger is completed (the “Atlantic Union closing share value”) by the fraction of a share (rounded to the nearest one-thousandth when expressed in decimal form) of Atlantic Union common stock that such stockholder would otherwise be entitled to receive.

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What will holders of Atlantic Union common stock receive at the effective time?

Holders of Atlantic Union common stock will not receive any consideration at the effective time, and their shares of Atlantic Union common stock will remain outstanding and will constitute shares of the surviving corporation. Following the effective time, shares of Atlantic Union common stock will continue to be listed on the NYSE under the trading symbol “AUB.”

Will the value of the merger consideration change between the date of this joint proxy statement/prospectus and the effective time?

Yes. Although the number of shares of Atlantic Union common stock that holders of Sandy Spring common stock will receive is fixed, the value of the merger consideration will fluctuate between the date of this joint proxy statement/prospectus and the effective time based on the market value of Atlantic Union common stock. Any fluctuation in the market price of Atlantic Union common stock after the date of this joint proxy statement/prospectus will change the value of the shares of Atlantic Union common stock that holders of Sandy Spring common stock will receive. Neither Atlantic Union nor Sandy Spring is permitted to terminate the merger agreement as a result, in and of itself, of any increase or decrease in the market price of Atlantic Union common stock or Sandy Spring common stock.

How will the merger affect the Sandy Spring equity awards?

At the effective time:

Restricted Stock Units: Each time-vesting restricted stock unit award of Sandy Spring (each, a “Sandy Spring RSU Award”) that is vested as of immediately prior to the effective time of the merger or held by a former employee, officer, director or other service provider or a non-employee director, whether or not vested immediately prior to the effective time, will fully vest and be cancelled and converted automatically into the right to receive the merger consideration. Each other Sandy Spring RSU Award that is outstanding immediately prior to the effective time will be assumed by Atlantic Union and will be converted into a restricted stock unit award that settles in a number of shares of Atlantic Union common stock determined by multiplying the number of shares of Sandy Spring common stock subject to the Sandy Spring RSU Award immediately prior to the effective time by the exchange ratio (each, an “Assumed RSU Award”), rounded down to the nearest whole share. Each Assumed RSU Award will continue to have, and will be subject to, the same terms and conditions as applied to the corresponding Sandy Spring RSU Award immediately prior to the effective time;

Performance-Based Restricted Stock Units: Each performance-vesting restricted stock unit award of Sandy Spring (each, a “Sandy Spring PSU Award”) that is held by a former employee, officer, director or other service provider will fully vest (based on target performance or, solely to the extent expressly set forth in the applicable award agreement with respect thereto, based on the greater of target performance and actual performance as of the effective time, as determined by the compensation committee of the Sandy Spring board of directors in good faith consultation with Atlantic Union (such applicable performance level the “Applicable Performance Level”)) and be cancelled and converted automatically into the right to receive the merger consideration, or in the case of each applicable accrued dividend equivalent unit with respect to such terminating Sandy Spring PSU Award, in an equivalent cash amount to the fair market value of the Sandy Spring common stock at the effective time. Each other Sandy Spring PSU Award will be assumed by Atlantic Union and converted into a time-vesting restricted stock unit award with respect to the number of shares of Atlantic Union common stock determined by multiplying the number of shares of Sandy Spring common stock subject to the Sandy Spring PSU Award immediately prior to the effective time (based on target performance or, if expressly required by the terms governing the Sandy Spring PSU Award, the Applicable Performance Level) by the exchange ratio (each, an “Assumed PSU Award”). In addition, each accrued dividend equivalent unit with respect to an Assumed PSU Award (each, a “Sandy Spring Dividend Equivalent Unit”) will be assumed by Atlantic Union and will be converted into a dividend equivalent unit award (each, an “Assumed Dividend Equivalent Unit”) that settles in an amount of cash equal to the fair market value (determined by reference to the closing price of a share of Atlantic Union common stock on the trading day immediately preceding the settlement date) at the time of settlement of the number of shares of Atlantic Union

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common stock equal to the number of shares of Sandy Spring common stock underlying the Sandy Spring Dividend Equivalent Unit immediately prior to the effective time (based on target performance), multiplied by the exchange ratio, rounded down to the nearest whole share. Each Assumed PSU Award and Assumed Dividend Equivalent Unit will continue to have, and will be subject to, the same terms and conditions as applied to the corresponding Sandy Spring PSU Award and Sandy Spring Dividend Equivalent Unit (other than performance-based vesting conditions) immediately prior to the effective time;

Restricted Stock: At the effective time, each share of Sandy Spring restricted stock that is outstanding immediately prior to the effective time will fully vest and be converted automatically into the right to receive the merger consideration in respect of such share of Sandy Spring restricted stock; and

Stock Options: Each option to purchase Sandy Spring common stock (each, a “Sandy Spring Option”) that is outstanding immediately prior to the effective time, will be cancelled and converted automatically into the right to receive a number of shares of Atlantic Union common stock (if any) equal to the exchange ratio multiplied by the number of shares of Sandy Spring common stock underlying the Sandy Spring Option, less a number of shares of Sandy Spring common stock having a fair market value (determined by reference to the closing price of a share of Sandy Spring common stock on the trading day immediately preceding the closing date of the merger) equal to the aggregate exercise price applicable to such Sandy Spring Option. Each Sandy Spring Option for which the applicable per share exercise price exceeds the closing price of a share of Sandy Spring common stock on the trading day immediately preceding the closing date of the merger will be cancelled as of the effective time for no consideration.

How does the Atlantic Union board of directors recommend that I vote at the Atlantic Union special meeting?

The Atlantic Union board of directors unanimously recommends that you vote “FOR” the Atlantic Union merger proposal and “FOR” the Atlantic Union adjournment proposal.

In considering the recommendations of the Atlantic Union board of directors, holders of Atlantic Union common stock should be aware that Atlantic Union directors and executive officers may have interests in the merger that are different from, or in addition to, the interests of holders of Atlantic Union common stock generally. For a more complete description of these interests, see the information provided in the section entitled “The Merger — Interests of Atlantic Union’s Directors and Executive Officers in the Merger.”

How does the Sandy Spring board of directors recommend that I vote at the Sandy Spring special meeting?

The Sandy Spring board of directors unanimously recommends that the holders of Sandy Spring common stock vote “FOR” the Sandy Spring merger proposal, “FOR” the Sandy Spring compensation proposal and “FOR” the Sandy Spring adjournment proposal.

In considering the recommendations of the Sandy Spring board of directors, holders of Sandy Spring common stock should be aware that Sandy Spring directors and executive officers may have interests in the merger that are different from, or in addition to, the interests of holders of Sandy Spring common stock generally. For a more complete description of these interests, see the information provided in the section entitled “The Merger — Interests of Sandy Spring’s Directors and Executive Officers in the Merger.”

Who is entitled to vote at the Atlantic Union special meeting?

The record date for the Atlantic Union special meeting is [ ]. All holders of Atlantic Union common stock who held shares at the close of business on the record date for the Atlantic Union special meeting are entitled to receive notice of, and to vote at, the Atlantic Union special meeting.

Each holder of Atlantic Union common stock is entitled to cast one vote on each matter properly brought before the Atlantic Union special meeting for each share of Atlantic Union common stock

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that such holder owned of record as of the record date. As of the close of business on the record date for the Atlantic Union special meeting, there were [ ] outstanding shares of Atlantic Union common stock. Attendance at the Atlantic Union special meeting is not required to vote. See the section entitled “The Atlantic Union Special Meeting — Proxies” for instructions on how to vote your shares without attending the Atlantic Union special meeting.

Who is entitled to vote at the Sandy Spring special meeting?

The record date for the Sandy Spring special meeting is [ ]. All holders of Sandy Spring common stock who held shares at the close of business on the record date for the Sandy Spring special meeting are entitled to receive notice of, and to vote at, the Sandy Spring special meeting.

Each holder of Sandy Spring common stock is entitled to cast one vote on each matter properly brought before the Sandy Spring special meeting for each share of Sandy Spring common stock that such holder owned of record as of the record date. As of the close of business on the record date for the Sandy Spring special meeting, there were [ ] outstanding shares of Sandy Spring common stock. Attendance at the Sandy Spring special meeting is not required to vote. See the section entitled “The Sandy Spring Special Meeting — Proxies” for instructions on how to vote your shares without attending the Sandy Spring special meeting.

What constitutes a quorum for the Atlantic Union special meeting?

Shares representing a majority of all the votes entitled to be cast by holders of Atlantic Union common stock must be present or represented by proxy at the Atlantic Union special meeting to constitute a quorum for action on that matter at the Atlantic Union special meeting. In the event that a quorum is not present at the time the Atlantic Union special meeting is to be called, a majority of the shareholders present or so represented may adjourn the Atlantic Union special meeting to a later date and time (subject to applicable law and compliance with the terms of the Atlantic Union articles of incorporation and bylaws). If you fail to submit a proxy or to vote at the Atlantic Union special meeting, or fail to instruct your bank, broker, trustee or other nominee how to vote, your shares of Atlantic Union common stock will not be counted towards a quorum. Abstentions are considered present for purposes of establishing a quorum.

What constitutes a quorum for the Sandy Spring special meeting?

Shares representing a majority of all the votes entitled to be cast by holders of Sandy Spring common stock must be present or represented by proxy at the Sandy Spring special meeting to constitute a quorum for action on that matter at the Sandy Spring special meeting. In the event that a quorum is not present at the Sandy Spring special meeting, the affirmative vote of a majority of the votes so represented at the Sandy Spring special meeting may adjourn the Sandy Spring special meeting to a later date and time (subject to applicable law and compliance with the terms of the Sandy Spring articles of incorporation and bylaws). If you fail to submit a proxy or to vote at the Sandy Spring special meeting, or fail to instruct your bank, broker, trustee or other nominee how to vote, your shares of Sandy Spring common stock will not be counted towards a quorum. Abstentions are considered present for purposes of establishing a quorum.

If my shares of common stock are held in “street name” by my bank, broker, trustee or other nominee, will my bank, broker, trustee or other nominee vote my shares for me?

If you hold your shares in a stock brokerage account or if your shares are held by a bank, broker, trustee or other nominee (that is, in “street name”) and fail to give voting instructions, your bank, broker, trustee or other nominee will not vote those shares. This applies to shares of both Atlantic Union common stock and Sandy Spring common stock.

Please follow the voting instructions provided by your broker, bank, trustee or other nominee. Please note that you may not vote shares held in street name by returning a proxy card directly to Atlantic Union or Sandy Spring or by voting at either special meeting unless you provide a “legal proxy,” which you must obtain from your bank, broker, trustee or other nominee. Further, brokers who hold shares

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of Atlantic Union common stock or Sandy Spring common stock may not give a proxy to Atlantic Union or Sandy Spring to vote those shares on any of the Atlantic Union proposals or any of the Sandy Spring proposals, respectively, without specific instructions from their customers.

What vote is required for the approval of each proposal at the Atlantic Union special meeting?

Proposal 1: Atlantic Union merger proposal: approval of the Atlantic Union merger proposal requires the affirmative vote of a majority of all the votes entitled to be cast at the Atlantic Union special meeting. Shares of Atlantic Union common stock not present, and shares present and not voted, whether by broker non-vote, abstention or otherwise, will have the same effect as a vote “AGAINST” the Atlantic Union merger proposal;

Proposal 2: Atlantic Union adjournment proposal: approval of the Atlantic Union adjournment proposal requires the affirmative vote of the majority of the votes cast on the proposal. Shares of Atlantic Union common stock not present, and shares present and not voted, whether by broker non-vote, abstention or otherwise, will have no effect on the outcome of the Atlantic Union adjournment proposal.

What vote is required for the approval of each proposal at the Sandy Spring special meeting?

Proposal 1: Sandy Spring merger proposal: approval of the Sandy Spring merger proposal requires the affirmative vote of 66 2/3% of all the votes entitled to be cast by holders of Sandy Spring common stock on the proposal at the Sandy Spring special meeting. Shares of Sandy Spring common stock not present, and shares present and not voted, whether by broker non-vote, abstention or otherwise, will have the same effect as a vote “AGAINST” the Sandy Spring merger proposal;

Proposal 2: Sandy Spring compensation proposal: approval of the Sandy Spring compensation proposal requires the affirmative vote of the majority of the votes cast at the Sandy Spring special meeting, and shares present and not voted, whether by broker non-vote, abstention or otherwise, will have no effect on the outcome of the Sandy Spring compensation proposal; and

Proposal 3: Sandy Spring adjournment proposal: Approval of the Sandy Spring adjournment proposal requires the affirmative vote of the majority of the votes cast at the Sandy Spring special meeting, and shares present and not voted, whether by broker non-vote, abstention or otherwise, will have no effect on the outcome of the Sandy Spring adjournment proposal.

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upon the earlier of the termination of the merger agreement or the effective time. As of [ ], the record date for the Sandy Spring special meeting, the members of the board of directors of Sandy Spring owned and held the sole dispositive and voting power over shares of Sandy Spring common stock representing approximately [ ]% of the voting power represented by all issued and outstanding shares of Sandy Spring common stock. A copy of the support agreement is attached to this joint proxy statement/prospectus as Annex C.

Why am I being asked to consider and vote on the Sandy Spring compensation proposal?

Under SEC rules, Sandy Spring is required to seek a non-binding, advisory vote with respect to the compensation that may be paid or become payable to Sandy Spring’s named executive officers that is based on or otherwise relates to the merger.

What happens if the holders of Sandy Spring common stock do not approve, by non-binding, advisory vote, the compensation proposal?

The vote on the Sandy Spring compensation proposal is separate and apart from the votes to approve the other proposals being presented at the Sandy Spring special meeting. Because the vote on the Sandy Spring compensation proposal is advisory only, it will not be binding upon Sandy Spring, Atlantic Union, or the surviving corporation or affect their obligation to pay or provide the compensation contemplated by the compensation agreements and arrangements. Accordingly, the merger-related compensation will be paid to Sandy Spring’s named executive officers to the extent payable in accordance with the terms of their compensation agreements and arrangements even if the holders of Sandy Spring common stock do not approve the Sandy Spring compensation proposal.

What if I hold shares in both Atlantic Union and Sandy Spring?

If you hold shares of both Atlantic Union common stock and Sandy Spring common stock, you will receive two separate packages of proxy materials. A vote cast as a holder of Atlantic Union common stock will not count as a vote cast as a holder of Sandy Spring common stock, and a vote cast as a holder of Sandy Spring common stock will not count as a vote cast as a holder of Atlantic Union common stock. Therefore, please submit separate proxies for your shares of Atlantic Union common stock and your shares of Sandy Spring common stock.

How can I vote my shares while in attendance at my respective special meeting?

Record holders: Shares held directly in your name as the holder of record of Atlantic Union common stock or Sandy Spring common stock may be voted at the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable. If you choose to vote your shares of Atlantic Union common stock at the Atlantic Union special meeting via the Atlantic Union special meeting website, please follow the instructions on your proxy card. If you choose to vote your shares of Sandy Spring common stock at the Sandy Spring special meeting, please follow the instructions on your proxy card.

Shares in “street name”: If your shares of Atlantic Union common stock or Sandy Spring common stock are held in street name and you wish to vote your shares at the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable, you must have your specific 16-digit control number, which is included on your proxy card or the voting instruction form from your bank, broker, trustee or other nominee. Please contact your bank, broker, trustee or other nominee to obtain further instructions.

Atlantic Union plan holders: If you participate in Atlantic Union’s Employee Stock Ownership Plan, or ESOP, and your account has investments in shares of Atlantic Union common stock, you must provide voting instructions to the Plan trustee (the Trustee) by internet, telephone, or proxy card for the shares to be voted according to your instructions. The deadline to provide voting instructions for shares held in the ESOP is [ ] p.m. Eastern Time on [ ]. After the applicable deadline, you will not be able to submit voting instructions or change prior voting instructions for any shares held in the ESOP. If you do not vote your shares held in the ESOP, the Trustee will vote the shares allocated to your ESOP account in the same proportion as it votes the shares of ESOP participants who have voted,

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subject to the Trustee’s fiduciary duties. You cannot vote your ESOP shares during the special meeting. Your voting instructions to the Trustee will be held in strict confidence and will not be revealed to any employee or director of Atlantic Union.

Sandy Spring employee stockholders: If you participate in the Sandy Spring Bank 401(k) Plan (the “401(k) plan”), and your plan account holds shares of Sandy Spring common stock, you must provide voting instructions to Principal Trust Company, the trustee for the 401(k) plan by internet, telephone, or proxy card for the shares to be voted according to your instructions. Your voting instructions to the trustee will be held in strict confidence. The deadline to provide voting instructions for shares held in the 401(k) plan is [ ] p.m. Eastern Time on [ ]. After the voting instruction deadline, you will not be able to submit voting instructions or change prior voting instructions for any shares. If you do not direct the trustee how to vote the shares in your 401(k) plan account, the trustee will vote the shares in the 401(k) plan in the same proportion as the voting instructions it receives from other participants as of the voting instruction deadline.

Even if you plan to attend the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable, Atlantic Union and Sandy Spring each recommend that you vote your shares in advance as described below so that your vote will be counted if you later decide not to or become unable to attend your respective special meeting.

Additional information on attending the special meetings can be found under the section entitled “The Atlantic Union Special Meeting” and under the section entitled “The Sandy Spring Special Meeting.”

How can I vote my shares without attending my respective special meeting?

Whether you hold your shares directly as the holder of record of Atlantic Union common stock or Sandy Spring common stock or beneficially in “street name,” you may direct your vote by proxy without attending the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable.

If you are a record holder of Atlantic Union common stock or Sandy Spring common stock, you can vote by proxy over the Internet, by telephone or by mail by following the instructions provided in the enclosed proxy card. Please note that if you hold shares beneficially in “street name,” you should follow the voting instructions provided by your bank, broker, trustee or other nominee.

If you hold shares of Atlantic Union common stock through the ESOP, you must provide voting instructions to the Plan trustee (the “Trustee”) by internet, telephone, or proxy card for the shares to be voted according to your instructions. The deadline to provide voting instructions for shares held in the ESOP is [ ] p.m. Eastern Time on [ ]. In the absence of instructions from you on how to vote such shares, the Trustee will vote your shares in the same proportion that the other shares in the ESOP are voted, subject to the Trustee’s fiduciary duties.

If you participate in the Sandy Spring Bank 401(k) Plan, and your plan account holds shares of Sandy Spring common stock, you must provide voting instructions to Principal Trust Company, the trustee for the 401(k) plan by internet, telephone, or proxy card for the shares to be voted according to your instructions. Your voting instructions to the trustee will be held in strict confidence. The deadline to provide voting instructions for shares held in the 401(k) plan is at 11:59 p.m. Eastern Time on [ ]. After the voting instruction deadline, you will not be able to submit voting instructions or change prior voting instructions for any shares. If you do not direct the trustee how to vote the shares in your 401(k) plan account, the trustee will vote the shares in the 401(k) plan in the same proportion as the voting instructions it receives from other participants as of the voting instruction deadline.

If you intend to submit your proxy by telephone or via the Internet, you must do so by 11:59 p.m. Eastern Time, on the day before the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable. If you intend to submit your proxy by mail, your completed proxy card must be received prior to the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable.

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What do I need to do now?

After carefully reading and considering the information contained in this joint proxy statement/prospectus, please vote as soon as possible. If you hold shares of Atlantic Union common stock or Sandy Spring common stock, please respond by completing, signing and dating the accompanying proxy card and returning it in the enclosed postage-paid envelope, or by submitting your proxy by telephone or through the Internet, as soon as possible so that your shares may be represented at your meeting. Please note that if you hold shares beneficially in “street name,” you should follow the voting instructions provided by your bank, broker, trustee or other nominee and if you hold shares of Sandy Spring common stock in a retirement plan account that allows you to choose how to vote the shares in your account, you should follow the voting instructions provided to you by the plan trustee or administrator.

Why is my vote important?

If you do not vote, it will be more difficult for Atlantic Union or Sandy Spring to obtain the necessary quorum to hold its respective special meeting. In addition, your failure to submit a proxy or vote at the respective special meeting, or failure to instruct your bank, broker, trustee or other nominee how to vote, will have the same effect as a vote “AGAINST” the Atlantic Union merger proposal and the Sandy Spring merger proposal, as applicable.

The Atlantic Union merger proposal requires the affirmative vote of the holders of a majority of all the votes entitled to be cast by holders of Atlantic Union common stock on the proposal at the Atlantic Union special meeting. The Atlantic Union adjournment proposal requires the affirmative vote of the holders of a majority of the votes cast at the Atlantic Union special meeting. The Atlantic Union board of directors unanimously recommends that you vote “FOR” the Atlantic Union merger proposal and “FOR” the Atlantic Union adjournment proposal.

The Sandy Spring merger proposal requires the affirmative vote of 66 2/3% of all the votes entitled to be cast by holders of Sandy Spring common stock on the proposal at the Sandy Spring special meeting. The Sandy Spring compensation proposal requires the affirmative vote of a majority of the votes cast by holders of Sandy Spring common stock on the proposal at the Sandy Spring special meeting. The Sandy Spring adjournment proposal requires the affirmative vote of a majority of the votes cast by holders of Sandy Spring common stock on the proposal at the Sandy Spring special meeting. The Sandy Spring board of directors unanimously recommends that you vote “FOR” the Sandy Spring merger proposal, “FOR” the Sandy Spring compensation proposal and “FOR” the Sandy Spring adjournment proposal.

Can I change my vote after I have delivered my proxy or voting instruction card?

Yes. You can change your vote at any time before your proxy is voted at your respective special meeting. You can do this by:

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timely delivery of a written notice of revocation of your proxy to Sandy Spring’s or Atlantic Union’s corporate secretary, as applicable, before the date of the respective special meeting;

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signing and returning a subsequently dated proxy by 11:59 p.m. Eastern Time, on the day before the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable, except that your proxy must be submitted by 11:59 p.m. Eastern Time on the day that is five days before the Sandy Spring special meeting if you hold shares of Sandy Spring common stock in a retirement plan account;

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voting by telephone or the Internet at a later time; or

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attending and voting at the respective special meeting.

If you hold your shares of Atlantic Union common stock or Sandy Spring common stock through a bank, broker, trustee or other nominee, you should contact your bank, broker, trustee or other nominee to change your vote.

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Will Atlantic Union be required to submit the Atlantic Union merger proposal to its shareholders even if the Atlantic Union board of directors has withdrawn or modified its recommendation?

Yes. Unless the merger agreement is terminated before the Atlantic Union special meeting, Atlantic Union is required to submit the Atlantic Union merger proposal to its shareholders even if the Atlantic Union board of directors has withdrawn or modified its recommendation.

Will Sandy Spring be required to submit the Sandy Spring merger proposal to its stockholders even if the Sandy Spring board of directors has withdrawn or modified its recommendation?

Yes. Unless the merger agreement is terminated before the Sandy Spring special meeting, Sandy Spring is required to submit the Sandy Spring merger proposal to its stockholders even if the Sandy Spring board of directors has withdrawn or modified its recommendation.

What happens if I sell my shares of Atlantic Union common stock or Sandy Spring common stock after the record date but before the date of the Atlantic Union special meeting or the Sandy Spring special meeting?

The respective record dates for the Atlantic Union special meeting and the Sandy Spring special meeting are earlier than the date of the Atlantic Union special meeting and the Sandy Spring special meeting and the date that the merger is expected to be completed. If you transfer your shares of Atlantic Union common stock or Sandy Spring common stock after the applicable record date but before the date of the Atlantic Union special meeting or the Sandy Spring special meeting, you will retain your right to vote at the Atlantic Union special meeting or the Sandy Spring special meeting, as applicable, but holders of Sandy Spring common stock will not have the right to receive the merger consideration to be received by the holders of Sandy Spring common stock in the merger. In order to receive the merger consideration, you must hold your shares of Sandy Spring common stock through the effective time.

Are holders of Atlantic Union common stock or holders of Sandy Spring common stock entitled to appraisal or dissenters’ rights?

No. No appraisal or dissenters’ rights will be available to the holders of Atlantic Union common stock or the holders of Sandy Spring common stock in connection with the merger. For more information, see the section entitled “The Merger — Appraisal or Dissenters’ Rights in the Merger.”

Are there any risks that I should consider in deciding whether to vote for the approval of the Atlantic Union merger proposal or the Sandy Spring merger proposal, or the other proposals to be considered at the Atlantic Union special meeting or the Sandy Spring special meeting, respectively?

Yes. You should read and carefully consider the risk factors set forth herein in the section entitled “Risk Factors.” You also should read and carefully consider the risk factors of Atlantic Union and Sandy Spring contained in the documents that are incorporated by reference into this joint proxy statement/prospectus.

What are the material U.S. federal income tax consequences of the merger to holders of Sandy Spring common stock?

The merger has been structured to qualify as a reorganization for federal income tax purposes, and it is a condition to our respective obligations to complete the merger that Atlantic Union and Sandy Spring each receive a legal opinion to the effect that the merger will so qualify. Assuming the receipt and accuracy of these opinions, holders of Sandy Spring common stock generally will not recognize any gain or loss for U.S. federal income tax purposes on the exchange of their Sandy Spring common stock for Atlantic Union common stock in the merger, except for any gain or loss that may result from the receipt of cash instead of a fractional share of Atlantic Union common stock. You should be aware that the tax consequences to you of the merger may depend upon your own situation. In addition, you may be subject to state, local or non-U.S. tax laws that are not discussed in this joint proxy statement/prospectus. You should therefore consult with your own tax advisor for a full understanding of the

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tax consequences to you of the merger. For a more complete discussion of the material U.S. federal income tax consequences of the merger, see the section entitled “Material U.S. Federal Income Tax Consequences of the Merger.”

When is the merger expected to be completed?

Atlantic Union and Sandy Spring expect the merger to close by the end of the third quarter of 2025. However, neither Atlantic Union nor Sandy Spring can predict the actual date on which the merger will be completed, or if the merger will be completed at all, because completion is subject to conditions and factors outside the control of both Atlantic Union and Sandy Spring. Atlantic Union and Sandy Spring must obtain the approval of the holders of Atlantic Union common stock for the Atlantic Union merger proposal and of the holders of Sandy Spring common stock for the Sandy Spring merger proposal, as well as obtain necessary regulatory approvals and satisfy certain other closing conditions.

What are the conditions to completion of the merger?

The obligations of Atlantic Union and Sandy Spring to complete the merger are subject to the satisfaction or waiver of certain closing conditions contained in the merger agreement, including approval by the holders of Atlantic Union common stock of the Atlantic Union merger proposal and by holders of Sandy Spring common stock of the Sandy Spring merger proposal, the authorization for listing on the NYSE of the Atlantic Union common stock to be issued pursuant to the merger agreement, the receipt of required regulatory approvals and the expiration of statutory waiting periods without the imposition of any materially burdensome regulatory condition and the receipt of tax opinions. For more information, see the section entitled “The Transaction Agreements — Description of the Merger Agreement — Conditions to Completion of the Merger.”

What happens if the merger is not completed?

If the merger is not completed, holders of Sandy Spring common stock will not receive any merger consideration for their shares of Sandy Spring common stock in connection with the merger and Sandy Spring common stock will not be exchanged for Atlantic Union common stock in connection with the merger. Instead, Sandy Spring will remain an independent public company, Sandy Spring common stock will continue to be listed on the Nasdaq, and Atlantic Union will not complete the issuance of shares of Atlantic Union common stock pursuant to the merger agreement. In addition, if the merger agreement is terminated in certain circumstances, a termination fee of $56 million may be payable either by Atlantic Union to Sandy Spring or by Sandy Spring to Atlantic Union, as applicable. See the section entitled “The Transaction Agreements — Description of the Merger Agreement — Termination Fees” for a more detailed discussion of the circumstances under which a termination fee will be required to be paid.

Should I send in my stock certificates now?

No. Please do not send in your stock certificates with your proxy. After the merger is completed, an exchange agent mutually agreed upon by Atlantic Union and Sandy Spring will send you instructions for exchanging Sandy Spring stock certificates for the merger consideration. See the section entitled “The Transaction Agreements — Description of the Merger Agreement — Conversion of Shares; Exchange of Sandy Spring Stock Certificates” for more information.

What should I do if I receive more than one set of voting materials for the same special meeting?

If you hold shares of Atlantic Union common stock or Sandy Spring common stock in “street name” and also directly in your name as a holder of record or otherwise, or if you hold shares of Atlantic Union common stock or Sandy Spring common stock in more than one brokerage account, you may receive more than one set of voting materials relating to the same special meeting.

Record holders: For shares held directly, please complete, sign, date and return each proxy card (or cast your vote by telephone or Internet as provided on each proxy card) or otherwise follow the voting

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instructions provided in this joint proxy statement/prospectus in order to ensure that all of your shares of Atlantic Union common stock or Sandy Spring common stock are voted.

Shares in “street name”: For shares held in “street name” through a bank, broker, trustee or other nominee, you should follow the procedures provided by your bank, broker, trustee or other nominee to vote your shares.

Who can help answer my questions?

Atlantic Union shareholders: If you have any questions about the merger or how to submit your proxy or voting instruction card, or if you need additional copies of this joint proxy statement/prospectus, please contact Atlantic Union’s corporate secretary at the address or phone number indicated on the cover page of this joint proxy statement/prospectus, or Atlantic Union’s proxy solicitor, [ ] (“[ ]”), by calling toll-free at ([ ]) [ ]-[ ], or for banks and brokers, collect at ([ ]) [ ]-[ ], or by email at [ ].

Sandy Spring stockholders: If you have any questions about the merger or how to submit your proxy or voting instruction card, or if you need additional copies of this joint proxy statement/prospectus, please contact Sandy Spring’s corporate secretary at [ ] or [ ], by calling toll-free at ([ ]) [ ]-[ ], or Sandy Spring’s proxy solicitor, [ ] (“[ ]”), by calling toll-free at ([ ]) [ ]-[ ], or for banks and brokers, collect at ([ ]) [ ]-[ ], or by email at [ ].

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SUMMARY

This summary highlights selected information in this joint proxy statement/prospectus and may not contain all of the information that is important to you. You should carefully read this entire joint proxy statement/prospectus and the other documents we refer you to for a more complete understanding of the matters being considered at the special meetings. In addition, we incorporate by reference important business and financial information about Atlantic Union and Sandy Spring into this joint proxy statement/prospectus. You may obtain the information incorporated by reference into this joint proxy statement/prospectus without charge by following the instructions in the section entitled “Where You Can Find More Information.”

The Parties to the Merger (pages 64 and 65)

Atlantic Union Bankshares Corporation, the financial holding company for Atlantic Union Bank, is incorporated under the laws of the Commonwealth of Virginia and maintains its principal executive office in Glen Allen, Virginia. Atlantic Union Bank has branches and ATMs located throughout Virginia and in portions of Maryland and North Carolina. Certain non-bank financial services affiliates of Atlantic Union Bank include: Atlantic Union Equipment Finance, Inc., which provides equipment financing; Atlantic Union Financial Consultants, LLC, which provides brokerage services; and Union Insurance Group, LLC, which offers various lines of insurance products. Shares of Atlantic Union’s common stock are listed on the NYSE under the symbol “AUB.” Atlantic Union’s principal executive office is located at 4300 Cox Road, Glen Allen, Virginia 23060 and its telephone number is (804) 633-5031.

Sandy Spring Bancorp, Inc., the holding company for Sandy Spring Bank, is incorporated in the state of Maryland and maintains its principal executive office in Olney, Maryland. Sandy Spring Bank is a Maryland state-chartered trust company with commercial banking powers that offers a broad range of commercial banking, retail banking, mortgage and trust services throughout central Maryland, Northern Virginia, and the greater Washington, D.C. market. Through its trust department and its subsidiaries, West Financial Services, Inc. (“West Financial”) and SSB Wealth Management, Inc. (d/b/a Rembert Pendleton Jackson, “RPJ”), Sandy Spring Bank offers a comprehensive menu of investment management services. Sandy Spring’s common stock is traded on the Nasdaq Global Select Market (“Nasdaq”) under the symbol “SASR.” Sandy Spring’s principal executive office is located at 17801 Georgia Avenue, Olney, Maryland 20832 and its telephone number is (301) 774-6400.

The Merger and the Merger Agreement (pages 66 and 115)

The terms and conditions of the merger are contained in the merger agreement, a copy of which is attached as Annex A to this joint proxy statement/prospectus. You are encouraged to read the merger agreement carefully and in its entirety, as it is the primary legal document that governs the merger.

Structure of the Transactions

Subject to the terms and conditions of the merger agreement, Sandy Spring will merge with and into Atlantic Union, with Atlantic Union as the surviving entity. Immediately following the merger, Atlantic Union will cause Sandy Spring Bank to merge with and into Atlantic Union Bank (the “bank merger”), with Atlantic Union Bank continuing as the surviving bank in the bank merger.

At any time prior to the effective time, Atlantic Union and Sandy Spring may, by mutual agreement, change the method or structure of effecting the combination of Atlantic Union and Sandy Spring if and to the extent that they both deem such change to be necessary, appropriate or desirable; provided that (unless the merger agreement is so amended in accordance with the terms thereof) no such change will (i) alter or change the exchange ratio or the number of shares of Atlantic Union common stock received by holders of Sandy Spring common stock in exchange for each share of Sandy Spring common stock, (ii) adversely affect the tax treatment of Sandy Spring’s stockholders or Atlantic Union’s shareholders pursuant to the merger agreement, (iii) adversely affect the tax treatment of Sandy Spring or Atlantic Union pursuant to the merger agreement or (iv) materially impede or delay the completion of the transactions contemplated by the merger agreement in a timely manner.

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Merger Consideration

Each share of Sandy Spring common stock issued and outstanding immediately prior to the effective time, except for shares of restricted Sandy Spring common stock and shares of Sandy Spring common stock owned by Sandy Spring or Atlantic Union, will be converted into the right to receive 0.900 shares of Atlantic Union common stock. All shares of Sandy Spring common stock converted into the right to receive the merger consideration will no longer be outstanding and will automatically be cancelled and cease to exist as of the effective time. Atlantic Union will not issue any fractional shares of Atlantic Union common stock in connection with the merger. Instead, a former holder of Sandy Spring common stock who otherwise would be entitled to receive such fractional share of Atlantic Union common stock in the merger will instead receive an amount in cash (rounded to the nearest cent) based on the Atlantic Union closing share value.

Atlantic Union common stock is listed on the NYSE under the symbol “AUB,” and Sandy Spring common stock is listed on the Nasdaq under the symbol “SASR.” The following table shows the closing sale prices of Atlantic Union common stock and Sandy Spring common stock as reported on the NYSE and Nasdaq, respectively, on October 18, 2024, the last full trading day before the public announcement of the merger agreement, and on [ ], the last practicable trading day before the date of this joint proxy statement/prospectus. This table also shows the implied value of the merger consideration to be issued in exchange for each share of Sandy Spring common stock, which was calculated by multiplying the closing price of Atlantic Union common stock on those dates by the exchange ratio of 0.900.

	Atlantic Union Common Stock			Sandy Spring Common Stock			Implied Value of One Share of Sandy Spring Common Stock
October 18, 2024		$	38.81		$	32.61		$	34.929
[ ]		$	[ ]		$	[ ]		$	[ ]

For more information on the exchange ratio, see the sections entitled “The Merger — Terms of the Merger” and “The Transaction Agreements — Description of the Merger Agreement — Merger Consideration.”

Treatment of Sandy Spring Equity Awards (page 106 and 117)

At the effective time, outstanding equity awards will be treated as follows:

Restricted Stock Units: Each Sandy Spring RSU Award that is vested as of immediately prior to the effective time of the merger or held by a former employee, officer, director or other service provider or a non-employee director, whether or not vested immediately prior to the effective time, will fully vest and be cancelled and converted automatically into the right to receive the merger consideration. Each other Sandy Spring RSU Award that is outstanding immediately prior to the effective time will be assumed by Atlantic Union and will be converted into an Assumed RSU Award that settles in a number of shares of Atlantic Union common stock determined by multiplying the number of shares of Sandy Spring common stock subject to the Sandy Spring RSU Award immediately prior to the effective time by the exchange ratio, rounded down to the nearest whole share. Each Assumed RSU Award will continue to have, and will be subject to, the same terms and conditions as applied to the corresponding Sandy Spring RSU Award immediately prior to the effective time;

Performance-Based Restricted Stock Units: Each Sandy Spring PSU Award that is held by a former employee, officer, director or other service provider will fully vest (based on target performance or, solely to the extent expressly set forth in the applicable award agreement with respect thereto, the Applicable Performance Level) and be cancelled and converted automatically into the right to receive the merger consideration, or in the case of each applicable accrued dividend equivalent unit with respect to such terminating Sandy Spring PSU Award, in an equivalent cash amount to the fair market value of the Sandy Spring common stock at the effective time. Each other Sandy Spring PSU Award will be assumed by Atlantic Union and converted into an Assumed PSU Award with respect to the number of shares of Atlantic Union common stock determined by multiplying the number of shares of Sandy Spring common stock

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subject to the Sandy Spring PSU Award immediately prior to the effective time (based on target performance or, if expressly required by the terms governing the Sandy Spring PSU Award, the Applicable Performance Level) by the exchange ratio. In addition, each accrued Sandy Spring Dividend Equivalent Unit will be assumed by Atlantic Union and will be converted into an Assumed Dividend Equivalent Unit that settles in an amount of cash equal to the fair market value (determined by reference to the closing price of a share of Atlantic Union common stock on the trading day immediately preceding the settlement date) at the time of settlement of the number of shares of Atlantic Union common stock equal to the number of shares of Sandy Spring common stock underlying the Sandy Spring Dividend Equivalent Unit immediately prior to the effective time (based on target performance), multiplied by the exchange ratio, rounded down to the nearest whole share. Each Assumed PSU Award and Assumed Dividend Equivalent Unit will continue to have, and will be subject to, the same terms and conditions as applied to the corresponding Sandy Spring PSU Award and Sandy Spring Dividend Equivalent Unit (other than performance-based vesting conditions) immediately prior to the effective time;

Stock Options: Each Sandy Spring Option that is outstanding immediately prior to the effective time, will be cancelled and converted automatically into the right to receive a number of shares of Atlantic Union common stock (if any) equal to the exchange ratio multiplied by the number of shares of Sandy Spring common stock underlying the Sandy Spring Option, less a number of shares of Sandy Spring common stock having a fair market value (determined by reference to the closing price of a share of Sandy Spring common stock on the trading day immediately preceding the closing date of the merger) equal to the aggregate exercise price applicable to such Sandy Spring Option. Each Sandy Spring Option for which the applicable per share exercise price exceeds the closing price of a share of Sandy Spring common stock on the trading day immediately preceding the closing date of the merger will be cancelled as of the effective time for no consideration.

Material U.S. Federal Income Tax Consequences of the Merger (page 138)

The merger has been structured to qualify as a “reorganization” within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended (the “Code”), and it is a condition to our respective obligations to complete the merger that Atlantic Union and Sandy Spring each receive a legal opinion to the effect that the merger will so qualify. Assuming the receipt and accuracy of these opinions, holders of Sandy Spring common stock generally will not recognize any gain or loss for U.S. federal income tax purposes on the exchange of their Sandy Spring common stock for Atlantic Union common stock in the merger, except for any gain or loss that may result from the receipt of cash instead of a fractional share of Atlantic Union common stock.

You should be aware that the tax consequences to you of the merger may depend upon your own situation. In addition, you may be subject to state, local or non-U.S. tax laws that are not discussed in this joint proxy statement/prospectus. You should therefore consult with your own tax advisor for a full understanding of the tax consequences to you of the merger.

Atlantic Union’s Reasons for the Merger; Recommendation of the Atlantic Union Board of Directors (page 74)

The Atlantic Union board of directors has unanimously determined that the merger, the merger agreement and the transactions contemplated by the merger agreement are advisable and in the best interests of Atlantic Union and its shareholders and has unanimously adopted and approved the merger agreement, the merger and the other transactions contemplated by the merger agreement. The Atlantic Union board of directors unanimously recommends that holders of Atlantic Union common stock vote “FOR” the Atlantic Union merger proposal and “FOR” the Atlantic Union adjournment proposal. For a more detailed discussion of the Atlantic Union board of directors’ recommendation, see the section entitled “The Merger — Atlantic Union’s Reasons for the Merger; Recommendation of the Atlantic Union Board of Directors.”

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Sandy Spring’s Reasons for the Merger; Recommendation of the Sandy Spring Board of Directors (page 87)

The Sandy Spring board of directors has unanimously determined that the merger, the merger agreement and the transactions contemplated by the merger agreement are advisable and in the best interests of Sandy Spring and its stockholders and has unanimously adopted and approved the merger agreement, the merger and the other transactions contemplated by the merger agreement. The Sandy Spring board of directors unanimously recommends that holders of Sandy Spring common stock vote “FOR” the Sandy Spring merger proposal, “FOR” the Sandy Spring compensation proposal and “FOR” the Sandy Spring adjournment proposal. For a more detailed discussion of the Sandy Spring board of directors’ recommendation, see the section entitled “The Merger — Sandy Spring’s Reasons for the Merger; Recommendation of the Sandy Spring Board of Directors.”

Opinion of Atlantic Union’s Financial Advisor (page 76)

Atlantic Union retained Morgan Stanley to provide it with financial advisory services in connection with a potential acquisition of Sandy Spring. Atlantic Union selected Morgan Stanley to act as its financial advisor based on Morgan Stanley’s qualifications, expertise and reputation and its knowledge of the business and affairs of Atlantic Union. On October 20, 2024, at a meeting of the Atlantic Union board of directors, Morgan Stanley rendered its oral opinion, subsequently confirmed by delivery of a written opinion dated October 20, 2024, to the effect that, as of the date of such opinion, and based upon and subject to the various assumptions made, procedures followed, matters considered, and qualifications and limitations on the scope of review undertaken by Morgan Stanley as set forth therein, the exchange ratio pursuant to the merger agreement was fair from a financial point of view to Atlantic Union.

The full text of the written opinion of Morgan Stanley, dated October 20, 2024, is attached as Annex D and incorporated by reference into this proxy statement/prospectus. The opinion sets forth, among other things, the assumptions made, procedures followed, matters considered and qualifications and limitations on the scope of the review undertaken by Morgan Stanley in rendering its opinion. Morgan Stanley’s opinion was directed to the Atlantic Union Board, in its capacity as such, and addresses only the fairness from a financial point of view to Atlantic Union of the exchange ratio pursuant to the merger agreement as of the date of the opinion. Morgan Stanley’s opinion does not address any other aspect of the transactions contemplated by the merger agreement and does not constitute a recommendation as to how the shareholders of Atlantic Union or Sandy Spring should vote at any shareholders’ meetings held in connection with the merger or any other matter or whether to take any other action with respect to the merger. The summary of Morgan Stanley’s opinion set forth in this proxy statement/prospectus is qualified in its entirety by reference to the full text of the opinion.

For a description of the opinion of Morgan Stanley, see the section entitled “The Merger — Opinion of Atlantic Union’s Financial Advisor.”

Opinion of Sandy Spring’s Financial Advisor (page 91)

In connection with the merger, Sandy Spring’s financial advisor, Keefe, Bruyette & Woods, Inc. (“KBW”), delivered a written opinion, dated October 20, 2024, to the Sandy Spring board of directors as to the fairness, from a financial point of view and as of the date of KBW’s opinion, to the holders of Sandy Spring common stock of the exchange ratio. The full text of KBW’s opinion, which describes the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by KBW in preparing the opinion, is attached as Annex E to this document. The opinion was for the information of, and was directed to, the Sandy Spring board of directors (in its capacity as such) in connection with its consideration of the financial terms of the merger. The opinion did not address the underlying business decision of Sandy Spring to engage in the merger or enter into the merger agreement or constitute a recommendation to the Sandy Spring board of directors in connection with the merger, and it does not constitute a recommendation to any holder of Sandy Spring common stock or any shareholder of any other entity as to how to vote in connection with the merger or any other matter.

Appraisal or Dissenters’ Rights in the Merger (page 114)

Holders of Atlantic Union common stock are not entitled to appraisal or dissenters’ rights under Virginia law and holders of Sandy Spring common stock are not entitled to appraisal or dissenters’ rights

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under Maryland law in connection with the merger. For more information, see the section entitled “The Merger — Appraisal or Dissenters’ Rights in the Merger.”

Interests of Atlantic Union’s Directors and Executive Officers in the Merger (page 106)

In considering the recommendation of the Atlantic Union board of directors to vote the Atlantic Union merger proposal and the Atlantic Union adjournment proposal, holders of Atlantic Union common stock should be aware that the directors and executive officers of Atlantic Union may have interests in the merger that are different from, or in addition to, the interests of holders of Atlantic Union common stock generally. The Atlantic Union board of directors was aware of these interests and considered them, among other matters, in making its recommendation that Atlantic Union shareholders vote to approve the Atlantic Union merger proposal and the Atlantic Union adjournment proposal.

These interests include 14 legacy Atlantic Union directors will continue to serve on the Atlantic Union board of directors, as further described in the section entitled “The Merger — Governance of Atlantic Union After the Merger.”

The Atlantic Union board of directors was aware of and considered these respective interests when deciding to adopt and approve the merger agreement and the other transaction agreements. For more information, see the section entitled “The Merger — Interests of Atlantic Union’s Directors and Executive Officers in the Merger.”

Interests of Sandy Spring’s Directors and Executive Officers in the Merger (page 106)

In considering the recommendation of Sandy Spring’s board of directors to vote for the Sandy Spring merger proposal, the Sandy Spring compensation proposal and the Sandy Spring adjournment proposal, holders of Sandy Spring common stock should be aware that the directors and executive officers of Sandy Spring may have interests in the merger that are different from, or in addition to, the interests of holders of Sandy Spring common stock generally. The Sandy Spring board of directors was aware of these interests and considered them, among other matters, in making its recommendation that holders of Sandy Spring common stock vote to approve the Sandy Spring merger proposal, the Sandy Spring compensation proposal and the Sandy Spring adjournment proposal.

These interests include:

•

Each Sandy Spring RSU Award that is vested as of immediately prior to the effective time of the merger or held by a non-employee director of Sandy Spring, whether or not vested immediately prior to the effective time, will fully vest and be cancelled and converted automatically into the right to receive the merger consideration. Each Sandy Spring RSU Award held by an executive officer that is outstanding immediately prior to the effective time will be assumed by Atlantic Union and will be converted into a restricted stock unit award that settles in a number of shares of Atlantic Union common stock determined by multiplying the number of shares of Sandy Spring common stock subject to the Sandy Spring RSU Award immediately prior to the effective time by the exchange ratio, rounded down to the nearest whole share.

•

Each Sandy Spring PSU Award that is held by current executive officers of Sandy Spring will be assumed by Atlantic Union and converted into a time-vesting restricted stock unit award with respect to the number of shares of Atlantic Union common stock determined by multiplying the number of shares of Sandy Spring common stock subject to the Sandy Spring PSU Award immediately prior to the effective time (based on target performance or, if expressly required by the terms governing the Sandy Spring PSU Award, the Applicable Performance Level) by the exchange ratio. In addition, each Sandy Spring Dividend Equivalent Unit will be assumed by Atlantic Union and will be converted into an Assumed Dividend Equivalent Unit that settles in an amount of cash equal to the fair market value (determined by reference to the closing price of a share of Atlantic Union common stock on the trading day immediately preceding the settlement date) at the time of settlement of the number of shares of Atlantic Union common stock equal to the number of shares of Sandy Spring common stock underlying the Sandy Spring Dividend Equivalent Unit immediately prior to the effective time (based on target performance), multiplied by the exchange ratio, rounded down to the nearest whole share.

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•

At the effective time, each share of Sandy Spring restricted stock held by a director or executive officer that is outstanding immediately prior to the effective time will fully vest and be converted automatically into the right to receive the merger consideration in respect of such share of Sandy Spring restricted stock.

•

Each Sandy Spring Option held by a director or executive officer that is outstanding immediately prior to the effective time will be cancelled and converted automatically into the right to receive a number of shares of Atlantic Union common stock (if any) equal to the exchange ratio multiplied by the number of shares of Sandy Spring common stock underlying the Sandy Spring Option, less a number of shares of Sandy Spring common stock having a fair market value (determined by reference to the closing price of a share of Sandy Spring common stock on the trading day immediately preceding the closing date of the merger) equal to the aggregate exercise price applicable to such Sandy Spring Option.

•

Sandy Spring’s executive officers are participants in Sandy Spring’s Executive Severance Plan, which provides for severance payments and benefits in the event of a qualifying termination of employment in connection with a change in control such as the merger.

•

Each of Sandy Spring’s executive officers maintains a split-dollar agreement with Sandy Spring that provides for a death benefit prior to separation from services, or after separation from service (other than for cause), following a change in control.

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Daniel J, Schrider has entered into a Consulting Agreement with Atlantic Union and will serve as a special advisor to Atlantic Union for a two-year period beginning at the effective time.

•

Three Sandy Spring directors, including Daniel J. Schrider, will be appointed to serve on the board of directors of Atlantic Union and Atlantic Union Bank as of the effective time of the merger, as further described in the section entitled “The Merger — Governance of Atlantic Union After the Merger.”

•

Sandy Spring’s directors and executive officers are entitled to continued indemnification and insurance coverage under the merger agreement.

The Sandy Spring board of directors was aware of and considered these respective interests when deciding to adopt and approve the merger agreement and in recommending to holders of Sandy Spring common stock that they vote to approve the Sandy Spring merger proposal, the Sandy Spring compensation proposal and the Sandy Spring adjournment proposal. For more information, see the section entitled “The Merger — Interests of Sandy Spring’s Directors and Executive Officers in the Merger.”

Governance of Atlantic Union After the Merger (page 111)

Governing Documents

The Atlantic Union articles of incorporation as in effect immediately prior to the effective time and the Atlantic Union bylaws as in effect immediately prior to the effective time will be the articles of incorporation and bylaws, respectively, of the surviving corporation, until thereafter amended in accordance with applicable law.

Board of Directors and Committees of the Board of Directors

Pursuant to the merger agreement, the Atlantic Union board of directors as of the effective time will have seventeen (17) members, consisting of:

•

fourteen (14) members of the Atlantic Union board of directors as of immediately prior to the effective time (the “legacy Atlantic Union directors”); and

•

Daniel J. Schrider and two other members of the Sandy Spring board of directors as of immediately prior to the effective time (the “legacy Sandy Spring directors”); provided that the legacy Sandy Spring directors must meet (i) the written director qualification and eligibility criteria of the nominating and corporate governance committee of the Atlantic Union board of directors and (ii) any applicable

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requirements or standards that may be imposed by a regulatory agency for service on the Atlantic Union board of directors (the “eligibility criteria”).

Prior to the effective time, Atlantic Union and Sandy Spring will cooperate in good faith (coordinating through the respective chairmen of the boards of directors of Atlantic Union and Sandy Spring) to mutually agree on the selection of the other legacy Sandy Spring directors (in addition to Mr. Schrider) who will join the Atlantic Union board of directors, Atlantic Union Bank board of directors and their respective committee appointments. In addition, as of the effective time, the legacy Sandy Spring directors will be appointed to the Atlantic Union Bank board of directors; provided that any such legacy Sandy Spring director must meet the eligibility criteria with respect to service on the Atlantic Union Bank board of directors.

Regulatory Approvals (page 111)

Subject to the terms of the merger agreement, Atlantic Union and Sandy Spring have agreed to cooperate with each other and use reasonable best efforts to promptly (and, in the case of the applications, notices, petitions and filings required to obtain the requisite regulatory approvals within 45 days of the date of the merger agreement) prepare and file all documentation to obtain as promptly as practicable all permits, consents, orders, approvals, waivers, non-objections and authorizations of all third parties and governmental entities which are necessary or advisable to consummate the transactions contemplated by the merger agreement (including the merger and the bank merger), and to comply with the terms and conditions of all such permits, consents, orders, approvals, waivers, non-objections and authorizations of all such governmental entities. These approvals include the approval of the Board of Governors of the Federal Reserve System (the “Federal Reserve Board”).

Although neither Atlantic Union nor Sandy Spring knows of any reason why it cannot obtain these regulatory approvals in a timely manner, Atlantic Union and Sandy Spring cannot be certain when or if they will be obtained, or that the granting of these regulatory approvals will not involve the imposition of conditions on the completion of the merger or the bank merger.

Expected Timing of the Merger

Atlantic Union and Sandy Spring expect the merger to close by the end of the third quarter of 2025. However, neither Atlantic Union nor Sandy Spring can predict the actual date on which the merger will be completed, or if the merger will be completed at all, because completion is subject to conditions and factors outside the control of both companies. Atlantic Union must first obtain the approval of holders of Atlantic Union common stock for the merger agreement and Atlantic Union share issuance, Sandy Spring must obtain the approval of holders of Sandy Spring common stock for the merger agreement, and both parties must obtain necessary regulatory approvals and satisfy certain other closing conditions.

The Transaction Agreements (page 115)

Merger Agreement

Conditions to Completion of the Merger

As more fully described in this joint proxy statement/prospectus and in the merger agreement, the completion of the merger depends on a number of conditions being satisfied or, where legally permissible, waived. These conditions include:

•

(i) approval of the merger agreement and the Atlantic Union share issuance by the holders of Atlantic Union common stock by the requisite Atlantic Union vote and (ii) approval of the merger agreement by the holders of Sandy Spring common stock by the requisite Sandy Spring vote;

•

the authorization for listing on the NYSE, subject to official notice of issuance, of the shares of Atlantic Union common stock that will be issuable pursuant to the merger agreement;

•

the effectiveness of the registration statement of which this joint proxy statement/prospectus forms a part, and the absence of any stop order suspending the effectiveness of the registration statement or proceedings for such purpose initiated or threatened by the SEC and not withdrawn;

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•

(i) the specified governmental consents and approvals, including from the Federal Reserve Board, certain state banking, securities and/or insurance regulatory authorities and under the Hart-Scott-Rodino Improvements Act of 1976, as amended (the “HSR Act”), having been received and remaining in full force and effect, and, the termination or expiration of all statutory waiting periods in respect thereof, and (ii) in each case that have not resulted in a materially burdensome regulatory condition;

•

no order, injunction or decree issued by any court or governmental entity of competent jurisdiction or other legal restraint or prohibition preventing the consummation of the merger or the bank merger being in effect, and no law, statute, rule, regulation, order, injunction or decree having been enacted, entered, promulgated or enforced by any governmental entity which prohibits or makes illegal the consummation of the merger or the bank merger;

•

the accuracy of the representations and warranties of Sandy Spring, on the one hand, and Atlantic Union, on the other hand, contained in the merger agreement, generally as of the date on which the merger agreement was entered into and as of the closing date, subject to the materiality standards provided in the merger agreement (and the receipt by each party of a certificate dated as of the closing date and signed on behalf of the other party by its chief executive officer or chief financial officer to such effect);

•

the performance by Sandy Spring, on the one hand, and Atlantic Union, on the other hand, in all material respects of the obligations, covenants and agreements required to be performed by it under the merger agreement at or prior to the closing date (and the receipt by each party of a certificate dated as of the closing date and signed on behalf of the other party by its chief executive officer or chief financial officer to such effect); and

•

receipt by Atlantic Union and Sandy Spring of opinions of legal counsel to the effect that on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code.

Termination of the Merger Agreement

The merger agreement may be terminated at any time prior to the effective time, whether before or after the receipt of the requisite Atlantic Union vote or the requisite Sandy Spring vote (except as indicated below), in the following circumstances:

•

by mutual written consent of Atlantic Union and Sandy Spring;

•

by either Atlantic Union or Sandy Spring if any governmental entity that must grant a requisite regulatory approval has denied approval of the merger or the bank merger and such denial has become final and nonappealable or any governmental entity of competent jurisdiction has issued a final and nonappealable order, injunction, decree or other legal restraint or prohibition permanently enjoining or otherwise prohibiting or making illegal the consummation of the merger or the bank merger, unless the failure to obtain a requisite regulatory approval is due to the failure of the party seeking to terminate the merger agreement to perform or observe its obligations, covenants and agreements set forth in the merger agreement or any other breach by such party of the merger agreement;

•

by either Atlantic Union or Sandy Spring if the merger has not been consummated on or before the termination date, unless the failure of the closing to occur by such date is due to the failure of the party seeking to terminate the merger agreement to perform or observe its obligations, covenants and agreements set forth in the merger agreement or any other breach by such party of the merger agreement;

•

by either Atlantic Union or Sandy Spring (provided that the terminating party is not then in material breach of any representation, warranty, obligation, covenant or other agreement contained in the merger agreement) if there is a breach of any of the obligations, covenants or agreements or any of the representations or warranties (or if any such representation or warranty ceases to be true) set forth in the merger agreement on the part of Sandy Spring, in the case of a termination by Atlantic Union, or Atlantic Union, in the case of a termination by Sandy Spring, which breach or failure to be true, either individually or in the aggregate with all other breaches by such party (or failures of such representations or warranties to be true), would constitute, if occurring or continuing on the

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closing date, the failure of an applicable closing condition of the terminating party and which is not cured within 45 days following written notice to the other party, or by its nature or timing cannot be cured during such period (or such fewer days as remain prior to the termination date);

•

by Sandy Spring, prior to the receipt of the requisite Atlantic Union vote, if (i) Atlantic Union or the Atlantic Union board of directors has made a recommendation change (as defined below) or (ii) Atlantic Union or the Atlantic Union board of directors breaches in any material respect its obligations relating to non-solicitation of acquisition proposals or its obligations related to the approval of the Atlantic Union shareholders and the Atlantic Union board recommendation;

•

by Atlantic Union, prior to the receipt of the requisite Sandy Spring vote, if (i) Sandy Spring or the Sandy Spring board of directors has made a recommendation change (as defined below) or (ii) Sandy Spring or the Sandy Spring board of directors breaches in any material respect its obligations relating to non-solicitation of acquisition proposals or its obligations related to the approval of the Sandy Spring stockholders and the Sandy Spring board recommendation; or

•

by either Atlantic Union or Sandy Spring, if (i) the requisite Atlantic Union vote has not been obtained upon a vote thereon taken at the Atlantic Union special meeting (including any adjournment or postponement thereof) or (ii) the requisite Sandy Spring vote has not been obtained upon a vote thereon taken at the Sandy Spring special meeting (including any adjournment or postponement thereof)

Neither Atlantic Union nor Sandy Spring is permitted to terminate the merger agreement as a result, in and of itself, of any increase or decrease in the market price of Atlantic Union common stock or Sandy Spring common stock.

Termination Fees

If the merger agreement is terminated by either Atlantic Union or Sandy Spring under certain circumstances, including circumstances involving alternative acquisition proposals and recommendation changes by Atlantic Union or Sandy Spring or their respective boards, Atlantic Union or Sandy Spring may be required to pay a termination fee of $56 million to the other party.

Support Agreements

Concurrently with the execution and delivery of the merger agreement, each of the members of the board of directors of Sandy Spring entered into a support agreement pursuant to which, among other things, each of the members of the board of directors of Sandy Spring agreed, subject to the terms of the support agreement, to (i) vote the shares of Sandy Spring common stock over which he or she has the sole power to vote or direct the voting of in favor of the approval of the merger agreement, and against any competing transaction and (ii) not transfer any such shares of Sandy Spring common stock prior to the Sandy Spring special meeting, with certain limited exceptions. The support agreements will terminate upon the earlier of the termination of the merger agreement or the effective time. As of [ ], the record date for the Sandy Spring special meeting, the members of the board of directors of Sandy Spring owned and held the sole dispositive and voting power over shares of Sandy Spring common stock representing approximately

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[ ]% of the voting power represented by all issued and outstanding shares of Sandy Spring common stock. A copy of the support agreement is attached to this joint proxy statement/prospectus as Annex C.

Accounting Treatment (page 111)

Atlantic Union and Sandy Spring each prepare their respective financial statements in accordance with accounting principles generally accepted in the United States (“GAAP”). The merger will be accounted for as an acquisition of Sandy Spring by Atlantic Union under the acquisition method of accounting in accordance with GAAP.

The Rights of Sandy Spring’s Stockholders Will Change as a Result of the Merger (page 145)

The rights of Sandy Spring stockholders are governed by Maryland law and by the Sandy Spring articles of incorporation and Sandy Spring bylaws. At the effective time, Sandy Spring stockholders will become Atlantic Union shareholders, and their rights will be governed by Virginia law and the Atlantic Union articles of incorporation and the Atlantic Union bylaws. Sandy Spring stockholders will have different rights once they become Atlantic Union shareholders due to differences between the Sandy Spring governing documents and Maryland law, on the one hand, and the Atlantic Union governing documents and Virginia law, on the other hand. These differences are described in more detail in the section entitled “Comparison of Shareholders’ Rights.”

Listing of Atlantic Union Common Stock; Delisting and Deregistration of Sandy Spring Common Stock (pages 113)

Following the merger, shares of Atlantic Union common stock will continue to be listed on the NYSE. Following the merger, Sandy Spring common stock will be delisted from the Nasdaq and deregistered under the Exchange Act.

The Atlantic Union Special Meeting (page 50)

The Atlantic Union special meeting will be held virtually via the Atlantic Union special meeting website, on [ ] at [ ] a.m. Eastern Time. At the Atlantic Union special meeting, holders of Atlantic Union common stock will be asked to consider and vote on the following proposals:

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the Atlantic Union merger proposal; and

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the Atlantic Union adjournment proposal.

You may vote at the Atlantic Union special meeting if you owned shares of Atlantic Union common stock at the close of business on [ ]. On that date, there were [ ] shares of Atlantic Union common stock outstanding, of which approximately [ ]% of the Atlantic Union common stock were owned and entitled to be voted by Atlantic Union directors and executive officers and their affiliates. We currently expect that Atlantic Union’s directors and executive officers will vote their shares in favor of the Atlantic Union merger proposal and the other proposals to be considered at the Atlantic Union special meeting, although only Atlantic Union directors have entered into the Atlantic Union support agreement obligating them to do so. As of [ ], the record date for the Atlantic Union special meeting, Atlantic Union’s directors owned and held sole dispositive and voting power over shares of Atlantic Union common stock representing approximately [ ]% of the voting power represented by all issued and outstanding shares of Atlantic Union common stock.

The Atlantic Union merger proposal will be approved if a majority of all the votes entitled to be cast by holders of Atlantic Union common stock present in person or represented by proxy at the Atlantic Union special meeting are voted in favor of such proposal. The Atlantic Union adjournment proposal will be approved if a majority of all the votes cast by holders of Atlantic Union common stock present in person or represented by proxy at the Atlantic Union special meeting are voted in favor of such proposal. If you mark “ABSTAIN” on your proxy, fail to submit a proxy or vote at the Atlantic Union special meeting or fail to instruct your bank, broker, trustee or other nominee how to vote with respect to the Atlantic Union merger proposal, it will have the same effect as a vote against approval of the proposal. If you mark “ABSTAIN” on your proxy, fail to submit a proxy or vote at the Atlantic Union special meeting or fail to

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instruct your bank, broker, trustee or other nominee how to vote with respect to the Atlantic Union adjournment proposal, it will have no effect on such proposal.

The Sandy Spring Special Meeting (page 57)

The Sandy Spring special meeting will be held on [ ] at [ ] a.m. Eastern Time. At the Sandy Spring special meeting, holders of Sandy Spring common stock will be asked to consider and vote on the following proposals:

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the Sandy Spring merger proposal;

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the Sandy Spring compensation proposal; and

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the Sandy Spring adjournment proposal.

You may vote at the Sandy Spring special meeting if you owned shares of Sandy Spring common stock at the close of business on [ ]. On that date, there were [ ] shares of Sandy Spring common stock outstanding, of which approximately [ ]% of the Sandy Spring common stock were owned and entitled to be voted by Sandy Spring directors and executive officers and their affiliates. We currently expect that Sandy Spring’s directors and executive officers will vote their shares in favor of the Sandy Spring merger proposal and the other proposals to be considered at the Sandy Spring special meeting, although only Sandy Spring directors have entered into the Sandy Spring support agreement obligating them to do so. As of [ ], the record date for the Sandy Spring special meeting, Sandy Spring’s directors owned and held sole dispositive and voting power over shares of Sandy Spring common stock representing approximately [ ]% of the voting power represented by all issued and outstanding shares of Sandy Spring common stock.

The Sandy Spring merger proposal will be approved if 66 2/3% of all the votes entitled to be cast by holders of Sandy Spring common stock at the Sandy Spring special meeting are voted in favor of such proposal. The Sandy Spring compensation proposal will be approved if a majority of all the votes cast by holders of Sandy Spring common stock present in person or represented by proxy are voted in favor of such proposal. The Sandy Spring adjournment proposal will be approved if a majority of all the votes cast by holders of Sandy Spring common stock present in person or represented by proxy at the Sandy Spring special meeting are voted in favor of such proposal. If you mark “ABSTAIN” on your proxy, fail to submit a proxy or vote at the Sandy Spring special meeting or fail to instruct your bank, broker, trustee or other nominee how to vote with respect to the Sandy Spring merger proposal, it will have the same effect as a vote against approval of the proposal. If you mark “ABSTAIN” on your proxy, fail to submit a proxy or vote at the Sandy Spring special meeting or fail to instruct your bank, broker, trustee or other nominee how to vote with respect to the Sandy Spring compensation proposal or the Sandy Spring adjournment proposal, it will have no effect on such proposal.

Forward Sale Agreement (page 140)

Atlantic Union entered into forward sale agreements on October 21, 2024, in each case with Morgan Stanley & Co. LLC, as forward purchaser (the “forward purchaser”), relating to an aggregate of 11,338,028 shares of Atlantic Union common stock (such forward sale agreements together, the “forward sale agreement”). In connection with the forward sale agreement, the forward purchaser or its affiliate borrowed from third parties an aggregate of 11,338,028 shares of Atlantic Union common stock.

For more information, see the section entitled “Description of Atlantic Union Capital Stock — Atlantic Union Common Stock — Forward Sale Agreement.”

Risk Factors (page 26)

In evaluating the merger agreement, the merger or the issuance of shares of Atlantic Union common stock, you should carefully read this joint proxy statement/prospectus and give special consideration to the

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RISK FACTORS

An investment by Sandy Spring’s stockholders in Atlantic Union common stock as a result of the exchange of shares of Atlantic Union common stock for shares of Sandy Spring common stock in the merger involves certain risks. Similarly, a decision on the part of Atlantic Union shareholders to approve the Atlantic Union merger proposal also involves risks for Atlantic Union shareholders. Certain material risks and uncertainties connected with the merger agreement and the transactions contemplated thereby, including the merger, the bank merger and ownership of Atlantic Union common stock are discussed below. In addition, Atlantic Union and Sandy Spring discuss certain other material risks connected with the ownership of Atlantic Union common stock and with Atlantic Union’s business, and with the ownership of Sandy Spring common stock and Sandy Spring’s business, respectively, under the caption “Risk Factors” appearing in their Annual Reports on Form 10-K most recently filed with the SEC and may include additional or updated disclosures of such material risks in their subsequent Quarterly Reports on Form 10-Q and Current Reports on Form 8-K that have been filed with the SEC or may be filed with the SEC after the date of this joint proxy statement/prospectus, each of which report is or will be incorporated by reference in this joint proxy statement/prospectus.

Holders of Sandy Spring common stock and holders of Atlantic Union common stock should carefully read and consider all of these risks and all other information contained in this joint proxy statement/prospectus, including the discussions of risk factors included in the documents incorporated by reference in this joint proxy statement/prospectus, in deciding whether to vote for approval of the various proposals for which they may be entitled to vote at the Sandy Spring special meeting or the Atlantic Union special meeting described herein. The risks described in this joint proxy statement/prospectus and in those documents incorporated by reference may adversely affect the value of Atlantic Union common stock that you, as an existing Atlantic Union shareholder, currently hold or that you, as an existing Sandy Spring stockholder, will hold upon the completion of the merger, and could result in a significant decline in the value of Atlantic Union common stock and cause the current holders of Atlantic Union common stock and/or the holders of Sandy Spring common stock to lose all or part of their respective investments in Atlantic Union common stock.

The dilution caused by the issuance of shares of Atlantic Union’s common stock in connection with the merger may adversely affect the market price of Atlantic Union’s common stock.

The dilution caused by the issuance of the new shares of Atlantic Union common stock to Sandy Spring stockholders in connection with the payment of the merger consideration may result in fluctuations in the market price of Atlantic Union common stock, including a stock price decrease.

Combining Atlantic Union and Sandy Spring may be more difficult, costly or time consuming than expected, and Atlantic Union may not realize the anticipated benefits of the acquisition.

A successful integration of Sandy Spring’s business with Atlantic Union’s business will depend substantially on the ability to consolidate operations, corporate cultures, systems and procedures and to eliminate redundancies and costs. Atlantic Union may not be able to combine each company’s business without encountering difficulties that could adversely affect the ability to maintain relationships with existing clients, customers, depositors and employees, such as:

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the loss of key employees;

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the disruption of operations and business;

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inability to maintain and increase competitive presence;

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loan and deposit attrition, customer loss and revenue loss;

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possible inconsistencies in standards, control procedures and policies;

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additional costs or unexpected problems with operations, personnel, technology and credit; and/or

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problems with the assimilation of new operations, systems, sites or personnel, which could divert resources from regular banking operations.

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Any disruption to the businesses could cause customers to remove their accounts and move their business to a competing financial institution. Integration efforts between the two companies may also divert management attention and resources. Additionally, general market and economic conditions or governmental actions affecting the financial industry generally may inhibit the successful integration of Atlantic Union and Sandy Spring.

Further, Atlantic Union and Sandy Spring entered into the merger agreement with the expectation that the acquisition of Sandy Spring by Atlantic Union will result in various benefits including, among other things, benefits relating to enhanced revenues, a strengthened market position for the surviving corporation, cross selling opportunities, technological efficiencies, cost savings and operating efficiencies. Achieving the anticipated benefits of the transactions contemplated by the merger agreement is subject to a number of uncertainties, including whether the integration is completed in an efficient, effective and timely manner, and general competitive factors in the marketplace. Failure to achieve these anticipated benefits on the anticipated timeframe, or at all, could result in a reduction in the price of Atlantic Union common stock as well as in increased costs, decreases in the amount of expected revenues and diversion of management’s time and energy and could materially and adversely affect Atlantic Union’s business, financial condition and operating results. Additionally, upon consummation of the transactions contemplated by the merger agreement, Atlantic Union will make fair value estimates of certain assets and liabilities in recording the acquisition. Actual values of these assets and liabilities could differ from such estimates, which could result in Atlantic Union not achieving the anticipated benefits of the acquisition. Finally, any cost savings that are realized may be offset by losses in revenues or other charges to earnings.

Atlantic Union and Sandy Spring have, and Atlantic Union following the closing will, incur significant transaction and transaction-related costs in connection with the transactions contemplated by the merger agreement.

Atlantic Union and Sandy Spring have incurred and expect to incur significant non-recurring costs associated with combining the operations of Atlantic Union and Sandy Spring. These costs include legal, financial advisory, accounting, consulting and other advisory fees, severance/employment-related costs, public company filing fees and other regulatory fees, printing costs and other related costs. Atlantic Union and Sandy Spring have begun collecting information in order to formulate detailed integration plans to deliver anticipated cost savings; however, many of the costs that will be incurred are, by their nature, difficult to estimate accurately. Additional unanticipated costs may be incurred in the integration of the businesses of Atlantic Union and Sandy Spring, and there are many factors beyond Atlantic Union’s or Sandy Spring’s control that could affect the total amount or timing of integration costs. Although Atlantic Union and Sandy Spring expect that the elimination of duplicative costs, as well as the realization of other efficiencies related to the integration of the businesses, may offset incremental transaction and transaction-related costs over time, this net benefit may not be achieved in the near term, or at all.

Whether or not the merger is consummated, Atlantic Union and Sandy Spring will incur substantial expenses in pursuing the merger and may adversely impact Atlantic Union’s and Sandy Spring’s earnings. The completion of the merger is conditioned upon customary closing conditions, including the receipt of required governmental authorizations, consents, orders and approvals, including approval by certain federal banking regulators and required approvals from Atlantic Union shareholders and Sandy Spring stockholders. Atlantic Union and Sandy Spring intend to pursue all required approvals in accordance with the merger agreement. However, these approvals could be delayed or not obtained at all, and there can be no assurance that such approvals will be obtained without additional cost, on the anticipated timeframe, or at all.

Regulatory approvals for the merger and the bank merger may not be received, may take longer than expected or may impose conditions that are not currently anticipated or that could have an adverse effect on Atlantic Union following the closing.

Before the merger and the bank merger may be completed, various approvals, consents and non-objections must be obtained from regulatory authorities. In determining whether to grant these approvals, regulatory authorities consider a variety of factors, including the regulatory standing of each party. These approvals could be delayed or not obtained at all, including due to any or all of the following: an adverse

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development in any party’s regulatory standing or any other factors considered by regulators in granting such approvals, governmental, political or community group inquiries, investigations or opposition; changes in legislation or the political environment, including as a result of changes of the U.S. executive administration, or Congressional leadership and regulatory agency leadership.

Even if the approvals are granted, they may impose terms and conditions, limitations, obligations or costs, or place restrictions on the conduct of the surviving corporation’s business or require changes to the terms of the transactions contemplated by the merger agreement. There can be no assurance that regulators will not impose any such conditions, limitations, obligations or restrictions or that such conditions, limitations, obligations or restrictions will not have the effect of delaying the completion of any of the transactions contemplated by the merger agreement, imposing additional material costs on or materially limiting the revenues of Atlantic Union following the closing or will otherwise reduce the anticipated benefits of the merger and the bank merger. In addition, there can be no assurance that any such conditions, limitations, obligations or restrictions will not result in the delay or abandonment of the merger and the bank merger. Additionally, the completion of the merger is conditioned on the absence of certain orders, injunctions or decrees by any governmental entity of competent jurisdiction that would prohibit or make illegal the completion of the merger and the bank merger.

Despite the parties’ expected commitment to use their reasonable best efforts to respond to any request for information and resolve any objection that may be asserted by any governmental entity with respect to the merger agreement, neither party is required under the terms of the merger agreement to take any action, commit to take any action, or agree to any condition or restriction in connection with obtaining these approvals, that would reasonably be expected to have a material adverse effect on the Atlantic Union and its subsidiaries, taken as a whole, after giving effect to the merger (measured on a scale relative only to the size of Sandy Spring and its subsidiaries, taken as a whole, without Atlantic Union and its subsidiaries).

Certain of Atlantic Union’s and Sandy Spring’s directors and executive officers may have interests in the merger that may differ from, or be in addition to, the interests of holders of Atlantic Union common stock and holders of Sandy Spring common stock generally.

Holders of Atlantic Union common stock and holders of Sandy Spring common stock should be aware that some of Atlantic Union’s and Sandy Spring’s directors and executive officers may have interests in the merger and have arrangements that are different from, or in addition to, those of holders of Atlantic Union common stock and holders of Sandy Spring common stock generally. These interests and arrangements may create potential conflicts of interest. The Atlantic Union board of directors and the Sandy Spring board of directors were aware of these respective interests and considered these interests, among other matters, when making their decisions to approve the merger agreement, the merger and the other transactions contemplated by the merger agreement, and in recommending that Atlantic Union shareholders vote to approve the merger agreement and approve the Atlantic Union share issuance, and that Sandy Spring stockholders vote to approve the merger agreement, as applicable. For a more complete description of these interests, please see the sections entitled “The Merger — Interests of Atlantic Union’s Directors and Executive Officers in the Merger” and “The Merger — Interests of Sandy Spring’s Directors and Executive Officers in the Merger.”

The merger agreement may be terminated in accordance with its terms and the merger may not be completed. Such failure to complete the merger could cause the results of Atlantic Union and Sandy Spring to be adversely affected, the stock prices of Atlantic Union and Sandy Spring to decline or have a material and adverse effect on the stock prices of Atlantic Union and Sandy Spring and each party’s results of operations.

If the merger is not completed for any reason, including as a result of Atlantic Union shareholders failing to approve the merger agreement or the Atlantic Union share issuance or Sandy Spring stockholders failing to approve the merger agreement, there may be various adverse consequences and Atlantic Union and/or Sandy Spring may experience negative reactions from the financial markets and from each party’s respective customers and employees. Certain costs related to the transactions contemplated by the merger agreement, such as legal, accounting and certain financial advisory fees, must be paid even if the merger is not completed. Moreover, Atlantic Union or Sandy Spring may be required to pay a termination fee of $56 million to the other party upon a termination of the merger agreement in certain circumstances. In

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addition, if the merger is not completed, whether because of the failure to receive required regulatory approvals in a timely fashion or because one of the parties has breached its obligations in a way that permits termination of the merger agreement, or for any other reason, Atlantic Union and Sandy Spring’s stock prices may decline to the extent that the current market price reflects a market assumption that the merger will be completed.

The market price for Atlantic Union common stock following the closing may be affected by factors different from those that historically have affected or currently affect Atlantic Union common stock and Sandy Spring common stock.

Subject to the terms and conditions of the merger agreement, at the effective time, holders of Sandy Spring common stock will receive shares of Atlantic Union common stock as merger consideration. Atlantic Union’s business and financial position will differ from the business and financial position of Atlantic Union and Sandy Spring before the completion of the merger and, accordingly, the results of operations of Atlantic Union will be affected by some factors that are different from those currently affecting Atlantic Union’s results of operations and those currently affecting the results of operations of Sandy Spring. Accordingly, the market price and performance of the surviving corporation’s common stock is likely to be different from the performance of Atlantic Union common stock or Sandy Spring common stock in the absence of the merger. In addition, general fluctuations in stock markets could have a material adverse effect on the market for, or liquidity of, Atlantic Union common stock or Sandy Spring common stock, regardless of actual operating performance.

The future results of Atlantic Union following the closing may suffer if Atlantic Union does not effectively manage its expanded operations.

Following the closing, the size of the business of Atlantic Union will increase significantly beyond the current size of either Atlantic Union’s or Sandy Spring’s business. Atlantic Union’s future success will depend, in part, upon its ability to manage this expanded business, which may pose challenges for management, including challenges related to the management and monitoring of new operations and associated increased costs and complexity. Atlantic Union may also face increased scrutiny from governmental authorities as a result of the significant increase in the size of its business.

Additionally, the internal policies of Atlantic Union Bank and Sandy Spring Bank with regards to their investment portfolios may differ on factors such as hold limits per bond issuer, life of the bond or credit risk appetite.

The benefits of the contemplated sale of certain commercial real estate loans following the closing may not be realized.

Atlantic Union Bank is contemplating a sale of a portfolio of commercial real estate loans (including loans held by Sandy Spring) (the “CRE loan portfolio”) concurrent with or after the closing of the merger. As a result, there are assets on the balance sheet of Sandy Spring Bank that the bank subsidiary of the surviving corporation is not expected to hold, based on differences internal policies and because of predetermined strategies to lower commercial real estate exposure, and Atlantic Union may dispose of such assets contemporaneous or subsequent to the closing. The disposition of certain assets in a high-interest rate environment, such as Atlantic Union has in the past experienced, is currently experiencing and may experience again in the future, could result in a sale of assets at a market price that is different than the estimated book value of such assets and impact regulatory capital ratios at the effective time. Further, Atlantic Union may replace such disposed assets with lower-yielding investments or to reduce wholesale borrowings at a lower cost than the yield on disposed assets, any of which could impact the surviving corporation’s future earnings and return on equity.

Atlantic Union and Sandy Spring will be subject to business uncertainties and contractual restrictions while the merger is pending.

Uncertainty about the effect of the merger on employees and customers may have an adverse effect on Atlantic Union and Sandy Spring. These uncertainties may impair Atlantic Union’s and Sandy Spring’s ability to attract, retain and motivate key personnel until the merger is completed, and could cause customers

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and others that deal with Atlantic Union and Sandy Spring to seek to change existing business relationships with Atlantic Union and Sandy Spring. In addition, subject to certain exceptions, Atlantic Union and Sandy Spring have agreed to operate their respective businesses in the ordinary course consistent with past practice in all material respects prior to the effective time, and Atlantic Union and Sandy Spring have agreed not to take certain actions, which could cause Atlantic Union and Sandy Spring to be unable to pursue other beneficial opportunities that may arise prior to the closing. See the section entitled “The Transaction Agreements — Description of the Merger Agreement — Covenants and Agreements.”

The shares of Atlantic Union common stock to be received by holders of Sandy Spring common stock as the merger consideration will have different rights from the shares of Sandy Spring common stock.

In the merger, holders of Sandy Spring common stock will become holders of Atlantic Union common stock and their rights as shareholders will be governed by Virginia law and the governing documents of Atlantic Union. See the sections entitled “Description of Atlantic Union Capital Stock” and “Comparison of Shareholders’ Rights.”

In connection with the merger, Atlantic Union will assume Sandy Spring’s outstanding debt obligations, and Atlantic Union’s level of indebtedness following the completion of the merger could adversely affect Atlantic Union’s ability to raise additional capital and to meet its obligations under its existing indebtedness.

In connection with the merger, Atlantic Union will assume Sandy Spring’s outstanding indebtedness. Atlantic Union’s existing debt, together with any future incurrence of additional indebtedness, could have important consequences for Atlantic Union’s creditors and Atlantic Union’s shareholders. For example, it could limit Atlantic Union’s ability to obtain additional financing for working capital, capital expenditures, debt service requirements, acquisitions and general corporate or other purposes; restrict Atlantic Union from making strategic acquisitions or cause Atlantic Union to make non-strategic divestitures; restrict Atlantic Union from paying dividends to its shareholders; increase Atlantic Union’s vulnerability to general economic and industry conditions; and require a substantial portion of cash flow from operations to be dedicated to the payment of principal and interest on Atlantic Union’s indebtedness, thereby reducing Atlantic Union’s ability to use cash flows to fund its operations, capital expenditures and future business opportunities following the merger.

Holders of Atlantic Union common stock and Sandy Spring common stock will have a reduced ownership and voting interest in the surviving corporation after the merger and will exercise less influence over management.

Holders of Atlantic Union common stock and Sandy Spring common stock currently have the right to vote in the election of the board of directors and on other matters affecting Atlantic Union and Sandy Spring, respectively. When the merger is completed, each holder of Sandy Spring common stock who receives shares of Atlantic Union common stock will become a holder of common stock of the surviving corporation, with a percentage ownership that is smaller than such holder’s percentage ownership of Sandy Spring. Based on the number of shares of Atlantic Union and Sandy Spring common stock outstanding as of the close of business on the respective record dates, and based on the number of shares of Atlantic Union common stock expected to be issued in the merger, the former holders of Sandy Spring common stock, as a group, are estimated to own approximately 29% of the fully diluted shares of Atlantic Union and the holders of Atlantic Union common stock as a group are estimated to own approximately 71% of the fully diluted shares of Atlantic Union, each as of immediately after the effective time. Additionally, Daniel J. Schrider and two other legacy Sandy Spring directors will join the Atlantic Union board of directors as of the effective time, and the Atlantic Union board of directors will be expanded to seventeen (17) directors. Because of this, holders of Sandy Spring common stock and Atlantic Union common stock may have less influence on the management and policies of the surviving corporation than they now have on the management and policies of Sandy Spring and Atlantic Union, respectively.

The merger agreement limits Atlantic Union’s and Sandy Spring’s respective ability to pursue alternatives to the merger and may discourage other companies from trying to acquire Atlantic Union or Sandy Spring.

The merger agreement contains “no shop” covenants that restrict each of Atlantic Union’s and Sandy Spring’s ability to, directly or indirectly, initiate, solicit, knowingly encourage or knowingly facilitate any

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inquiries or proposals with respect to any acquisition proposal, engage or participate in any negotiations with any person concerning any acquisition proposal, provide any confidential or nonpublic information or data to, or have or participate in any discussions with, any person relating to any acquisition proposal, subject to certain exceptions, grant any waiver, amendment or release of or under, or fail to enforce, any confidentiality, standstill, or similar agreement (or any confidentiality, standstill or similar provision of any other contract) or, unless the merger agreement has been terminated in accordance with its terms, approve or enter into any term sheet, letter of intent, commitment, memorandum of understanding, agreement in principle, acquisition agreement, merger agreement or other agreement in connection with or relating to any acquisition proposal.

The merger agreement further provides that, during the 12-month period following the termination of the merger agreement under specified circumstances, including the entry into a definitive agreement or consummation of a transaction with respect to an alternative acquisition proposal, Atlantic Union or Sandy Spring may be required to pay a termination fee of $56 million to the other party. See the section entitled “The Transaction Agreements — Description of the Merger Agreement — Termination Fees.”

These provisions could discourage a potential third-party acquirer that might have an interest in acquiring all or a significant portion of Atlantic Union or Sandy Spring from considering or proposing that acquisition.

The merger will not be completed unless important conditions are satisfied or waived, including approval of the merger agreement by holders of Sandy Spring common stock and approval of the merger agreement and approval of the share issuance by holders of Atlantic Union shareholders.

Specified conditions set forth in the merger agreement must be satisfied or waived to complete the merger. If the conditions are not satisfied or, subject to applicable law, waived, the merger will not occur or will be delayed and each of Sandy Spring and Atlantic Union may lose some or all of the intended benefits of the merger. The following conditions must be satisfied or waived, if permissible, before Sandy Spring and Atlantic Union are obligated to complete the merger:

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(i) approval of the merger agreement by the shareholders of Atlantic Union, including the issuance of shares of Atlantic Union common stock in the merger, by the requisite Atlantic Union vote and (ii) approval of the merger agreement by the stockholders of Sandy Spring by the requisite Sandy Spring vote;

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the authorization for listing on the NYSE, subject to official notice of issuance, of the shares of Atlantic Union common stock that will be issued pursuant to the merger agreement;

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the specified governmental consents and approvals, including from the Federal Reserve Board, having been obtained and the termination or expiration of all statutory waiting periods in respect thereof, in each case without a materially burdensome regulatory condition;

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no order, injunction or decree issued by any court or governmental entity of competent jurisdiction or other legal restraint or prohibition preventing the completion of either of the merger or the bank merger being in effect, and no law, statute, rule, regulation, order, injunction or decree having been enacted, entered, promulgated or enforced by any governmental entity which prohibits or makes illegal the completion of the merger or the bank merger;

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the accuracy of the representations and warranties of Sandy Spring on the one hand, and Atlantic Union, on the other hand, contained in the merger agreement, generally as of the date on which the merger agreement was entered into and as of the closing date, subject to the materiality standards provided in the merger agreement (and the receipt by each party of a certificate dated as of the closing date and signed on behalf of the other party by its chief executive officer or chief financial officer to such effect);

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the performance by Sandy Spring, on the one hand, and Atlantic Union, on the other hand, in all material respects of the obligations, covenants and agreements required to be performed by it under

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the merger agreement at or prior to the closing date (and the receipt by each party of a certificate dated as of the closing date and signed on behalf of the other party by its chief executive officer or chief financial officer to such effect); and

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receipt by each of Sandy Spring and Atlantic Union of opinions of legal counsel to the effect that on the basis of facts, representations and assumptions set forth or referred to in such opinion, the merger will qualify as a “reorganization” within the meaning of Section 368(a) of the Code.

The unaudited pro forma condensed combined financial information included in this joint proxy statement/ prospectus is preliminary and the actual financial condition and results of operations of the surviving corporation after the merger may differ materially.

The unaudited pro forma condensed combined financial information in this joint proxy statement/ prospectus is presented for illustrative purposes only and is not necessarily indicative of what the surviving corporation’s actual financial condition or results of operations would have been had the merger been completed on the dates indicated. The unaudited pro forma condensed combined financial information reflects adjustments, which are based upon preliminary estimates, to record the Sandy Spring identifiable assets acquired and liabilities assumed at fair value and the resulting goodwill recognized. The fair value estimates reflected in this joint proxy statement/prospectus are preliminary, and final amounts will be based upon the actual consideration and the fair value of the assets and liabilities of Sandy Spring as of the date of the completion of the merger. Accordingly, the final acquisition accounting adjustments may differ materially from the pro forma adjustments reflected in this joint proxy statement/prospectus. For more information, see the section entitled “Unaudited Pro Forma Condensed Combined Financial Information.”

Holders of Atlantic Union common stock and holders of Sandy Spring common stock will not have appraisal rights or dissenters’ rights in the merger.

Appraisal rights (also known as dissenters’ rights) are statutory rights that, if applicable under law, enable security holders to dissent from an extraordinary transaction, such as a merger, and to demand that the corporation pay the fair value for their shares as determined by a court in a judicial proceeding instead of receiving the consideration offered to security holders in connection with the extraordinary transaction.

Under Section 13.1-730 of the Virginia law, the holders of Atlantic Union common stock will not be entitled to appraisal or dissenters’ rights in connection with the merger with respect to any shares of Atlantic Union common stock that remain outstanding after the consummation of the merger. If the merger is completed, holders of Atlantic Union common stock will not receive any consideration for their shares of Atlantic Union common stock, and their shares of Atlantic Union common stock will remain outstanding and will constitute shares of the surviving corporation.

The holders of Sandy Spring common stock will not be entitled to dissenters’ or appraisal rights (or rights of an objecting stockholder under Section 3-201 et seq. of the MGCL or otherwise) with respect to the merger. Except as provided in the MGCL, a stockholder of a Maryland corporation has the right to demand and receive payment of the fair value of the stockholder’s stock from the successor if, among other things, (i) the corporation consolidates or merges with another corporation, (ii) the corporation amends its charter in a way which alters the contract rights, as expressly set forth in the charter, of any outstanding stock and substantially adversely affects the stockholder’s rights, unless the right to do so is reserved by the charter of the corporation, or (iii) the corporation is converted in accordance with the MGCL. However, under the MGCL, a stockholder of a Maryland corporation may not demand the fair value of the stockholder’s stock and is bound by the terms of the transaction if, except in limited circumstances not applicable to the merger, any shares of the applicable class or series of the corporation’s stock are listed on a national securities exchange on the record date for determining stockholders entitled to vote on the transaction objected to. On [ ], the record date for the Sandy Spring special meeting, shares of Sandy Spring common stock were listed on the Nasdaq.

Neither of the opinions regarding the fairness, from a financial point of view, of the exchange ratio delivered to the Sandy Spring board of directors or the Atlantic Union board of directors prior to the signing of the merger agreement reflect any changes in circumstances since the date on which such opinions were delivered.

The opinion, dated October 20, 2024, rendered by KBW, financial advisor to Sandy Spring, to the Sandy Spring board of directors, and the opinion, dated October 20, 2024, rendered by Morgan Stanley,

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financial advisor to Atlantic Union, to the Atlantic Union board of directors, were based upon the prospective financial information and other information made available to such financial advisors as of the date of each respective opinion. Neither opinion reflects any changes that may occur or may have occurred after the date on which each opinion was delivered, including changes to the operations and prospects of Sandy Spring or Atlantic Union, changes in general market and economic conditions, or other changes which may be beyond the control of Sandy Spring and Atlantic Union. Any such changes may alter the relative value of Sandy Spring or Atlantic Union or the prices of shares of Sandy Spring common stock or Atlantic Union common stock by the time the merger is completed. The opinions do not speak as of the date the merger will be completed or as of any date other than the date of each respective opinion. For a description of the opinion that the Sandy Spring board of directors received from Sandy Spring’s financial advisor, please see the section entitled “The Merger — Opinion of Sandy Spring’s Financial Advisor.” For a description of the opinion that the Atlantic Union board of directors received from Atlantic Union’s financial advisor, please see the section entitled “The Merger — Opinion of Atlantic Union’s Financial Advisor.”

Risks Relating to Atlantic Union’s Business

In addition to the below, you should read and consider risk factors specific to Atlantic Union’s business that will also affect Atlantic Union after the merger. These risks are described in the sections entitled “Risk Factors” in Atlantic Union’s Annual Report on Form 10-K for the year ended December 31, 2023, Atlantic Union’s Quarterly Report on Form 10-Q for the nine months ended September 30, 2024 and in other documents incorporated by reference into this joint proxy statement/prospectus. Please see the section entitled “Where You Can Find More Information” for the location of information incorporated by reference into this joint proxy statement/prospectus.

Settlement provisions contained in the forward sale agreement could result in substantial dilution to Atlantic Union’s earnings per share and return on equity or result in substantial cash payment obligations.

Morgan Stanley & Co. LLC, as forward purchaser under the forward sale agreement, has the right to accelerate the forward sale agreement and require Atlantic Union to physically settle on a date specified by the forward purchaser if:

•

it (or its affiliate) (i) is unable to borrow a number of shares of Atlantic Union common stock equal to the number of shares of Atlantic Union common stock underlying the forward sale agreement because of the lack of sufficient shares being made available for share borrowing by lenders or (ii) would incur a stock loan rate greater than the rate specified in the forward sale agreement to continue to borrow such shares;

•

certain ownership thresholds applicable to the forward purchaser, its affiliates and other persons who may form a beneficial share ownership group or whose ownership positions would be aggregated with the forward purchaser are exceeded;

•

Atlantic Union declares any dividend or distribution on Atlantic Union common stock that constitutes an extraordinary dividend or is payable in (i) cash in excess of a specified amount (other than extraordinary dividends), (ii) securities of another company owned (directly or indirectly) by Atlantic Union as a result of a spin-off or similar transaction or (iii) any other type of securities (other than Atlantic Union common stock), rights, warrants or other assets for payment at less than the prevailing market price, as reasonably determined by the forward purchaser;

•

there is an announcement of any event or transaction that, if consummated, would result in certain extraordinary events (as such term is defined in the forward sale agreement and which includes certain mergers (other than the merger or the bank merger) and tender offers and the delisting of Atlantic Union common stock); or

•

certain other events of default, termination events or other specified events occur, including, among other things, any material misrepresentation made by Atlantic Union in connection with entering into the forward sale agreement or the occurrence of a hedging disruption or a change in law (as such terms are defined in the forward sale agreement).

The forward purchaser’s decision to exercise its right to accelerate the settlement of the forward sale agreement will be made irrespective of Atlantic Union’s need for capital. In such cases, Atlantic Union

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could be required to issue and deliver shares of Atlantic Union common stock under the physical settlement provisions of the forward sale agreement irrespective of Atlantic Union’s capital needs, which would result in dilution to Atlantic Union’s earnings per share and return on equity.

Atlantic Union expects to physically settle the forward sale agreement (by the delivery of shares of Atlantic Union common stock) and receive proceeds from the sale of those shares of Atlantic Union common stock upon one or more forward settlement dates within approximately eighteen (18) months from the date of the forward sale agreement. Atlantic Union may also elect cash settlement or net share settlement for all or a portion of its obligations under the forward sale agreement. Upon physical settlement or, if Atlantic Union so elects, net share settlement of the forward sale agreement, delivery of shares of Atlantic Union common stock in connection with such physical settlement or (to the extent that Atlantic Union is obligated to deliver shares of Atlantic Union common stock) net share settlement will result in dilution to Atlantic Union’s earnings per share and return on equity. If Atlantic Union elects cash settlement or net share settlement with respect to all or a portion of the shares of Atlantic Union common stock underlying the forward sale agreement, then Atlantic Union expects that the forward purchaser (or an affiliate thereof) will purchase a number of shares of Atlantic Union common stock necessary to satisfy its or its affiliate’s obligation to return the shares of Atlantic Union common stock borrowed from third parties in connection with sales of shares of Atlantic Union common stock related to the forward sale agreement and, if applicable in connection with net share settlement, to deliver shares of Atlantic Union common stock to Atlantic Union. If the market value of Atlantic Union common stock at the time of such purchase (as determined pursuant to the terms of the forward sale agreement) is above the forward sale price under the forward sale agreement at that time, then Atlantic Union would pay or deliver, as the case may be, to the forward purchaser under the forward sale agreement, an amount in cash, or a number of shares of Atlantic Union common stock with a market value (as determined pursuant to the terms of the forward sale agreement), equal to such difference. Any such difference could be significant.

In addition, the purchase of shares of Atlantic Union common stock in connection with the forward purchaser or its affiliate unwinding its hedge positions could cause the price of Atlantic Union common stock to increase over such time (or reduce or prevent a decrease over such time), thereby increasing the amount of cash Atlantic Union would owe to the forward purchaser (or decreasing the amount of cash that the forward purchaser would owe Atlantic Union) upon a cash settlement of the forward sale agreement or increasing the number of shares of Atlantic Union common stock Atlantic Union would deliver to the forward purchaser (or decreasing the number of shares of Atlantic Union common stock that the forward purchaser would deliver to Atlantic Union) upon net share settlement of the forward sale agreement. Atlantic Union will not be able to control the manner in which the forward purchaser (or its affiliate) unwinds its hedge positions.

Moreover, the forward sale price that Atlantic Union expects to receive upon physical settlement of the forward sale agreement will be subject to increase or decrease based on the specified rate less a spread, and subject to price adjustment and other provisions of the forward sale agreement, including a decrease based on amounts related to expected dividends on Atlantic Union common stock on dates specified in the forward sale agreement and if the cost to the forward purchaser (or its affiliate) of borrowing a number of shares of Atlantic Union common stock underlying the forward sale agreement exceeds a specified amount. If the specified rate is less than the spread on any day, the interest rate factor will result in a daily reduction of the applicable forward sale price. Reductions in the applicable forward sale price could also increase the amount of cash Atlantic Union would owe to the forward purchaser (or decrease the amount of cash that the forward purchaser would owe Atlantic Union) upon a cash settlement of the forward sale agreement or increase the number of shares of Atlantic Union common stock Atlantic Union would deliver to the forward purchaser (or decrease the number of shares of Atlantic Union common stock that the forward purchaser would deliver to Atlantic Union) upon net share settlement of the forward sale agreement.

In case of Atlantic Union’s bankruptcy or insolvency, the forward sale agreement will automatically terminate, and Atlantic Union would not receive the proceeds from the forward sales of Atlantic Union common stock.

If Atlantic Union files for or consents to a proceeding seeking a judgment in bankruptcy or insolvency or any other relief under any bankruptcy or insolvency law or other similar law affecting creditors’ rights, or Atlantic Union or a regulatory authority with jurisdiction over Atlantic Union presents a petition for the

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winding-up or liquidation of Atlantic Union, and Atlantic Union consents to such a petition, then the forward sale agreement will automatically terminate. If the forward sale agreement so terminates under these circumstances, Atlantic Union would not be obligated to deliver to the forward purchaser any Atlantic Union common stock not previously delivered, and the forward purchaser would be discharged from its obligation to pay the applicable forward sale price per share in respect of any Atlantic Union common stock not previously settled under the forward sale agreement. Therefore, to the extent that there are any shares of Atlantic Union common stock with respect to which the forward sale agreement has not been settled at the time of the commencement of any such bankruptcy or insolvency proceedings, Atlantic Union would not receive the forward sale price per share in respect of those shares of Atlantic Union common stock.

Risks Relating to Sandy Spring’s Business

You should read and consider risk factors specific to Sandy Spring’s business that will also affect Atlantic Union after the merger. These risks are described in the sections entitled “Risk Factors” in Sandy Spring’s Annual Report on Form 10-K for the year ended December 31, 2023, Sandy Spring’s Quarterly Report on Form 10-Q for the nine months ended September 30, 2024 and in other documents incorporated by reference into this joint proxy statement/prospectus. Please see the section entitled “Where You Can Find More Information” for the location of information incorporated by reference into this joint proxy statement/ prospectus.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This joint proxy statement/prospectus contains “forward-looking statements” within the meaning of Section 27A of the Securities Act, and Rule 175 promulgated thereunder, and Section 21E of the Exchange Act, and Rule 3b-6 promulgated thereunder, which statements involve inherent risks and uncertainties. Any statements about Atlantic Union’s, Sandy Spring’s or the surviving corporation’s plans, objectives, expectations, strategies, beliefs, or future performance or events constitute forward-looking statements. Such statements are generally identified as those that include words or phrases such as “believes,” “expects,” “anticipates,” “plans,” “trend,” “objective,” “continue,” or similar expressions or future or conditional verbs such as “will,” “would,” “should,” “could,” “might,” “may,” or similar expressions. Forward-looking statements involve known and unknown risks, uncertainties, assumptions, estimates, and other important factors that change over time and could cause actual results to differ materially from any results, performance, or events expressed or implied by such forward-looking statements. Such forward-looking statements include but are not limited to statements about the benefits of the transactions contemplated by the merger agreement, including future financial and operating results, the surviving corporation’s plans, objectives, expectations and intentions, and other statements that are not historical facts.

These forward-looking statements are subject to risks and uncertainties that may cause actual results to differ materially from those projected. In addition to factors previously disclosed in Atlantic Union’s and Sandy Spring’s reports filed with the SEC, the following factors, among others, could cause actual results to differ materially from forward-looking statements or historical performance: the occurrence of any event, change, or other circumstance that could give rise to the right of one or both of the parties to terminate the merger agreement; the outcome of any legal proceedings that may be instituted against Atlantic Union or Sandy Spring; the possibility that the merger does not close when expected or at all because required regulatory, shareholder, or other approvals and other conditions to closing are not received or satisfied on a timely basis or at all (and the risk that such approvals may result in the imposition of conditions that could adversely affect the surviving corporation or the expected benefits of the transactions); the risk that the benefits from the transactions may not be fully realized or may take longer to realize than expected, including as a result of changes in, or problems arising from, general economic and market conditions, interest and exchange rates, monetary policy, laws and regulations and their enforcement, and the degree of competition in the geographic and business areas in which Atlantic Union and Sandy Spring operate; the ability to promptly and effectively integrate the businesses of Atlantic Union and Sandy Spring; the possibility that the transactions may be more expensive to complete than anticipated, including as a result of unexpected factors or events; reputational risk and potential adverse reactions of Atlantic Union’s or Sandy Spring’s customers, employees or other business partners, including those resulting from the announcement or completion of the transactions; the dilution caused by Atlantic Union’s issuance of additional shares of its capital stock in connection with the transaction; and the diversion of management’s attention and time from ongoing business operations and opportunities on matters related to the merger.

These factors are not necessarily all of the factors that could cause Atlantic Union’s, Sandy Spring’s or the surviving corporation’s actual results, performance, or achievements to differ materially from those expressed in or implied by any of the forward-looking statements. Other factors, including unknown or unpredictable factors, also could harm Atlantic Union’s, Sandy Spring’s or the surviving corporation’s results. Further information regarding Atlantic Union, Sandy Spring and factors which could affect the forward-looking statements contained herein can be found in the section entitled “Risk Factors” and those set forth in Atlantic Union’s and Sandy Spring’s annual reports and other filings with the SEC that are incorporated by referenced into this joint proxy statement/prospectus, as described in the section entitled “Where You Can Find More Information.”

For any forward-looking statements made in this joint proxy statement/prospectus or in any documents incorporated by reference into this joint proxy statement/prospectus, Atlantic Union and Sandy Spring claim the protection of the safe harbor for forward-looking statements contained in the Private Securities Litigation Reform Act of 1995. You are cautioned not to place undue reliance on the forward-looking statements, which speak only as of the date of this joint proxy statement/prospectus or the dates of the documents incorporated by reference in this joint proxy statement/prospectus. As for the forward-looking statements that relate to future financial results and other projections, actual results will be different due to the inherent uncertainties of estimates, forecasts and projections and may be better or worse than projected

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UNAUDITED PRO FORMA CONDENSED COMBINED FINANCIAL INFORMATION

Introduction

The following unaudited pro forma condensed combined financial data is based on the separate historical financial statements of Atlantic Union, Sandy Spring and American National Bankshares Inc. (“American National”) after giving effect to the merger and our completed acquisition of American National (the “American National acquisition”). The unaudited pro forma condensed combined financial data has been prepared in accordance with Article 11 of Regulation S-X and should be read in conjunction with the accompanying notes.

The unaudited pro forma condensed combined balance sheet as of September 30, 2024 combines the unaudited consolidated balance sheet of Atlantic Union as of September 30, 2024 with the unaudited consolidated balance sheet of Sandy Spring as of September 30, 2024, giving effect to the merger and the forward sale agreement as if the merger had been consummated and the forward sale agreement had been fully physically settled on September 30, 2024. See the section entitled “Description of Atlantic Union Capital Stock — Atlantic Union Common Stock — Forward Sale Agreement” for more information on the forward sale agreement.

The unaudited pro forma condensed combined statement of income for the year ended December 31, 2023, combines the audited consolidated statement of income of Atlantic Union for the year ended December 31, 2023, with the audited consolidated statement of income of American National for the year ended December 31, 2023, as well as the audited consolidated statement of income of Sandy Spring for the year ended December 31, 2023, giving effect to the American National acquisition, the merger and the forward sale agreement as if the American National acquisition and the merger had been consummated and the forward sale agreement had been fully physically settled on January 1, 2023.

The unaudited pro forma condensed combined statement of income for the nine months ended September 30, 2024, combines the unaudited consolidated statement of income of Atlantic Union for the nine months ended September 30, 2024, with the unaudited consolidated statement of income of American National for the three months ended March 31, 2024, as well as the unaudited consolidated statement of income of Sandy Spring for nine months ended September 30, 2024, giving effect to the American National acquisition, the merger and the forward sale agreement as if the American National acquisition and the merger had been consummated and the forward sale agreement had been fully physically settled on January 1, 2023.

The unaudited pro forma condensed combined financial data was derived from, and should be read in conjunction with, the following historical financial statements and the accompanying notes, which are incorporated by reference into this joint proxy statement/prospectus:

•

The historical audited consolidated financial statements of Atlantic Union as of and for the year ended December 31, 2023 (included in Atlantic Union’s Annual Report on Form 10-K for the fiscal year ended December 31, 2023);

•

The historical unaudited consolidated financial statements of Atlantic Union as of and for the nine months ended September 30, 2024 (included in Atlantic Union’s Quarterly Report on Form 10-Q for the fiscal quarter ended September 30, 2024);

•

The historical audited consolidated financial statements of Sandy Spring as of and for the year ended December 31, 2023 (included in Sandy Spring’s Annual Report on Form 10-K for the fiscal year ended December 31, 2023);

•

The historical unaudited consolidated financial statements of Sandy Spring as of and for the nine months ended September 30, 2024 (included in Sandy Spring’s Quarterly Report on Form 10-Q for the fiscal quarter ended September 30, 2024);

•

The historical audited consolidated financial statements of American National as of and for the year ended December 31, 2023 (included as Exhibit 99.1 in Atlantic Union’s Amended Current Report on Form 8-K/A dated April 18, 2024); and

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•

The historical unaudited consolidated financial statements of American National as of and for the three months ended March 31, 2024 (included as Exhibit 99.5 in Atlantic Union’s Current Report on Form 8-K dated October 21, 2024).

The unaudited pro forma condensed combined financial data should also be read together with other financial data included elsewhere or incorporated by reference into this joint proxy statement/prospectus, including the unaudited pro forma condensed combined financial statements of Atlantic Union and American National as of and for the year ended December 31, 2023, attached as Exhibit 99.2 to Atlantic Union’s Amended Current Report on Form 8-K/A dated April 18, 2024.

The foregoing historical financial statements have been prepared in accordance with generally accepted accounting principles in the United States (“U.S. GAAP”). The unaudited pro forma condensed combined financial data has been prepared based on the aforementioned historical financial statements and the assumptions and adjustments as described in the notes to the unaudited pro forma condensed combined financial data. The pro forma adjustments reflect transaction accounting adjustments related to the American National acquisition, the merger and the forward sale agreement, all of which are discussed in further detail below. Amounts presented reflect the accounting for the acquisitions of American National and Sandy Spring by Atlantic Union. The unaudited pro forma condensed combined financial data is presented for illustrative purposes only and does not purport to represent the surviving corporation’s consolidated results of operations or consolidated financial position that would actually have occurred had the American National acquisition and the merger been consummated and the forward sale agreement been fully physically settled on the dates assumed or to project the surviving corporation’s consolidated results of operations or consolidated financial position for any future date or period.

The unaudited pro forma condensed combined financial data appearing below is based on available preliminary information and certain assumptions that are believed to be reasonable as of the date of this joint proxy statement/prospectus and also does not consider any potential effects of changes in market conditions, certain asset dispositions (including the proposed sale of approximately $2 billion of commercial real estate loans at, or shortly after, the completion of the merger), cost savings, or revenue synergies, among other factors discussed in the sections entitled “Cautionary Statement Regarding Forward-Looking Statements,” “Risk Factors” and the consolidated audited and unaudited financial statements of Atlantic Union and Sandy Spring included elsewhere, or incorporated by reference, in this joint proxy statement/prospectus, and, accordingly, does not attempt to predict or suggest future results. In addition, as explained in more detail in the accompanying notes, the preliminary allocation of the pro forma purchase price reflected in the unaudited pro forma condensed combined financial data is subject to adjustment and may vary significantly from the actual purchase price allocation that will be recorded upon completion of the merger.

The American National Acquisition

On April 1, 2024, Atlantic Union completed its previously announced merger with American National, pursuant to the Agreement and Plan of Merger, dated as of July 24, 2023, by and between Atlantic Union and American National. At the effective time of the merger, American National merged with and into Atlantic Union, with Atlantic Union continuing as the surviving corporation. Immediately following the merger, American National Bank and Trust Company, American National’s wholly owned subsidiary bank, merged with and into Atlantic Union Bank, with Atlantic Union Bank continuing as the surviving bank. American National’s results of operations have been included in Atlantic Union’s consolidated results since the date of the American National acquisition.

The unaudited pro forma condensed combined financial information has been prepared using the acquisition method of accounting for business combinations under U.S. GAAP, with Atlantic Union as the acquirer for accounting purposes. Certain reclassifications have been made to the historical financial statements of American National to conform to the presentation in Atlantic Union’s financial statements. The unaudited pro forma condensed combined balance sheet as of September 30, 2024, does not reflect transaction accounting adjustments related to the American National acquisition as the American National acquisition is already reflected in the historical balance sheet of Atlantic Union as of September 30, 2024. The unaudited pro forma condensed combined statements of income for the nine months ended September 30, 2024, and for the year ended December 31, 2023, are presented as if the American National acquisition

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occurred on January 1, 2023, each of which does not necessarily indicate the results of operations if the businesses had been combined for the historical period, or the results of operations in future periods.

Accounting for the Merger

The unaudited pro forma condensed combined financial data is provided to aid stockholders in their analysis of the financial aspect of the merger and the American National acquisition and to provide information about the forward sale agreement. The unaudited pro forma condensed combined financial data has been prepared in accordance with Article 11 of Regulation S-X and should be read in conjunction with the accompanying notes.

Forward Sale Agreement

In connection with the forward sale agreement, the forward purchaser borrowed from third parties an aggregate of 11,338,028 shares of Atlantic Union common stock. Such borrowed shares of Atlantic Union common stock were delivered by the forward seller for sale to the underwriters in this offering. Atlantic Union did not initially receive any proceeds from the sale of the shares of Atlantic Union common stock sold by the forward seller to the underwriters. Atlantic Union expects to physically settle the forward sale agreement at the offering size of $402.5 million (by the delivery of shares of Atlantic Union common stock) and receive proceeds from the sale of those shares of Atlantic Union common stock upon one or more forward settlement dates within approximately 18 months from the date of the forward sale agreement. Atlantic Union expects to receive net proceeds from the full physical settlement of the forward sale agreement, before expenses, of approximately $386.4 million, based upon the initial forward sale price of $34.08 per share, which is equal to the public offering price per share, less the underwriting discount per share. Atlantic Union may also elect cash settlement or net share settlement for all or a portion of its obligations under the forward sale agreement. If Atlantic Union elects to cash settle or net share settle the forward sale agreement, then it may not receive any proceeds from the issuance of shares of Atlantic Union common stock in respect of the forward sale agreement, and Atlantic Union will instead receive or pay cash (in the case of cash settlement) or receive or deliver shares of Atlantic Union common stock (in the case of net share settlement).

Basis of Pro Forma Presentation

The historical financial data of Atlantic Union, American National and Sandy Spring has been adjusted to give pro forma effect to the transaction accounting required for the American National acquisition, the merger and the forward sale agreement. The adjustments in the unaudited pro forma condensed combined financial data have been identified and presented to provide relevant information necessary to evaluate the financial overview of the surviving corporation upon closing of the merger and full physical settlement of the forward sale agreement at the offering size of $402.5 million.

The unaudited pro forma condensed combined financial data is not necessarily indicative of what the surviving corporation’s balance sheet or statement of income would have been had the American National acquisition been completed, the merger been completed and the forward sale agreement been fully physically settled at the offering size of $402.5 million as of the dates indicated, nor do they purport to project the future financial position or operating results of the surviving corporation. The unaudited pro forma condensed combined financial data is presented for illustrative purposes only and does not reflect the costs of any integration activities or cost savings or synergies that may be achieved because of the merger. American National and Atlantic Union did not have any historical material relationship before the American National acquisition. Sandy Spring and Atlantic Union have not had any historical material relationship before the merger. Accordingly, no pro forma adjustments were required to eliminate activities among the companies.

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UNAUDITED PRO FORMA CONDENSED COMBINED BALANCE SHEET
AS OF SEPTEMBER 30, 2024

(Dollars in thousands, except share and per share data)

	As of September 30, 2024
(Dollars in thousands)	Atlantic Union (Historical)			Sandy Spring (As Reclassified)(1)			Transaction Accounting Adjustments			Note	Combined Pro Forma
Assets
Cash and cash equivalents		$	528,070		$	750,346		$	240,979	(2), (3)		$	1,519,395
Securities available for sale, at fair value			2,608,182			1,149,056			—				3,757,238
Securities held to maturity, at carrying value			807,080			220,296			(41,583)	(4)			985,793
Other investments, at cost			117,881			71,136			—				189,017
Loans held for sale			11,078			21,914			—				32,992
Loans held for investment, net of deferred fees and costs			18,337,299			11,491,921			(742,458)	(5)			29,086,762
Less: allowance for loan losses			160,685			131,428			40,951	(6)			333,064
Total loans held for investment, net			18,176,614			11,360,493			(783,409)				28,753,698
Premises and equipment, net			115,093			57,249			9,000	(7)			181,342
Goodwill			1,212,710			363,436			488,639	(8)			2,064,785
Amortizable intangibles, net			90,176			30,514			247,652	(9)			368,342
Bank owned life insurance			489,759			170,584			—				660,343
Other assets			647,080			188,049			122,550	(10)			957,679
Total assets		$	24,803,723		$	14,383,073		$	283,828			$	39,470,624
Liabilities
Noninterest-bearing demand deposits		$	4,422,909		$	2,903,063			—			$	7,325,972
Interest-bearing deposits			15,882,378			8,834,631			5,000	(11)			24,722,009
Total deposits			20,305,287			11,737,694			5,000				32,047,981
Other short-term borrowings			434,227			520,767			—				954,994
Long-term borrowings			417,937			371,251			(10,000)	(12)			779,188
Other liabilities			463,856			124,524			—				588,380
Total liabilities			21,621,307			12,754,236			(5,000)				34,370,543
Stockholders’ Equity
Preferred stock			173			—			—				173
Common stock			118,494			45,125			24,723	(2), (13), (14)			188,342
Additional paid-in capital			2,277,024			748,202			1,324,675	(2), (13), (14)			4,349,901
Retained earnings			1,079,032			911,411			(1,136,471)	(3), (6), (13)			853,972
Accumulated other comprehensive loss			(292,307)			(75,901)			75,901	(13)			(292,307)
Total stockholders’ equity			3,182,416			1,628,837			288,828				5,100,081
Total liabilities and stockholders’ equity		$	24,803,723		$	14,383,073		$	283,828			$	39,470,624

Please refer to the notes to the unaudited pro forma condensed combined financial data.

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UNAUDITED PRO FORMA CONDENSED COMBINED STATEMENTS OF INCOME
FOR THE NINE MONTHS ENDED SEPTEMBER 30, 2024

(Dollars in thousands, except share and per share data)

	For the Nine Months Ended September 30, 2024
(Dollars in thousands, except per share amounts)	Atlantic Union (Historical)			American National (Historical) (for the three months ended March, 31, 2024)			American National Transaction Accounting Adjustments			Note	Combined Pro Forma Subtotal			Sandy Spring (As reclassified)(15)			Sandy Spring Transaction Accounting Adjustments			Note	Combined Pro Forma
Interest and dividend income
Interest and fees on loans		$	810,886		$	28,339		$	15,007	(16)		$	854,232		$	456,309		$	149,154	(23)		$	1,459,695
Interest and dividends on securities:
Taxable			68,012			2,615			4,844	(17)			75,471			21,319			34,691	(24)			131,481
Nontaxable			24,455			24			17	(17)			24,496			5,385			9,284	(24)			39,165
Other interest income			4,977			625			—				5,602			18,212			—				23,814
Total interest and dividend income			908,330			31,603			19,868				959,801			501,225			193,129				1,654,155
Interest expense
Interest on deposits			354,584			10,871			—				365,455			227,062			—				592,517
Interest on borrowings			38,456			1,641			116	(18)			40,213			33,123			1,364	(26)			74,700
Total interest expense			393,040			12,512			116				405,668			260,185			1,364				667,217
Net interest income			515,290			19,091			19,752				554,133			241,040			191,765				986,938
Provision for credit losses			32,592			400			—				32,992			9,724			—				42,716
Net interest income after provision for credit losses			482,698			18,691			19,752				521,141			231,316			191,765				944,222
Noninterest income
Service charges on deposit accounts			27,447			518			—				27,965			8,765			—				36,730
Loss on sale of securities			(6,510)			—			—				(6,510)			—			—				(6,510)
Other operating income			62,714			3,755			(1,068)	(19),(22)			65,401			48,904			—				114,305
Total noninterest income			83,651			4,273			(1,068)				86,856			57,669			—				144,525
Noninterest expenses
Salaries and benefits			199,867			8,527			—				208,394			115,549			—				323,943
Occupancy expenses			22,267			1,555			—				23,822			14,278			—				38,100
Technology and data processing			28,138			1,461			—				29,599			9,414			—				39,013
Amortization of intangible assets			13,693			215			4,031	(20)			17,939			6,527			29,954	(28)			54,420
Merger-related costs			33,005			165			—				33,170			—			—				33,170
Other expenses			80,889			3,488			(411)	(22)			83,966			63,279			—				147,245
Total noninterest expenses			377,859			15,411			3,620				396,890			209,047			29,954				635,891
Income before income taxes			188,490			7,553			15,064				211,107			79,938			161,811				452,856
Income tax expense			37,144			1,509			3,465	(30)			42,118			20,550			37,217	(30)			99,885
Net income			151,346			6,044			11,599				168,989			59,388			124,594				352,971
Dividends on preferred stock			8,901			—			—				8,901			—			—				8,901
Net income available to common shareholders		$	142,445		$	6,044		$	11,599			$	160,088		$	59,388		$	124,594			$	344,070
Basic earnings per common share		$	1.68		$	0.57									$	1.32						$	2.32
Diluted earnings per common share		$	1.68		$	0.57									$	1.31						$	2.32
Basic weighted average number of common shares outstanding			84,933,126			10,630,663			3,720,732	(21)			99,284,521			45,046,255			3,997,691	(31)			148,328,467
Diluted weighted average number of common shares outstanding			84,933,213			10,630,663			3,720,732	(21)			99,284,608			45,156,521			3,987,869	(31)			148,428,998

Please refer to the notes to the unaudited pro forma condensed combined financial data.

TABLE OF CONTENTS

UNAUDITED PRO FORMA CONDENSED COMBINED STATEMENTS OF INCOME
FOR THE YEAR ENDED DECEMBER 31, 2023

(Dollars in thousands, except share and per share data)

	For the Year Ended December 31, 2023
(Dollars in thousands, except per share amounts)	Atlantic Union (Historical)			American National (Historical)			American National Transaction Accounting Adjustments			Note	Combined Pro Forma Subtotal			Sandy Spring (As reclassified)(15)			Sandy Spring Transaction Accounting Adjustments			Note	Combined Proforma
Interest and dividend income
Interest and fees on loans		$	846,923		$	106,471		$	43,467	(16)		$	996,861		$	579,960		$	198,873	(23)		$	1,775,694
Interest and dividends on securities:
Taxable			67,075			11,034			16,146	(17)			94,255			26,992			46,254	(24)			167,501
Nontaxable			34,381			139			57	(17)			34,577			7,224			12,379	(24)			54,180
Other interest income			6,071			2,585			—				8,656			23,348			—				32,004
Total interest and dividend income			954,450			120,229			59,670				1,134,349			637,524			257,506				2,029,379
Interest expense
Interest on deposits			296,689			28,843			—				325,532			225,028			(5,000)	(25)			545,560
Interest on borrowings			46,748			6,804			463	(18)			54,015			57,946			1,818	(26)			113,779
Total interest expense			343,437			35,647			463				379,547			282,974			(3,182)				659,339
Net interest income			611,013			84,582			59,207				754,802			354,550			260,688				1,370,040
Provision for credit losses			31,618			495			—				32,113			(17,561)			103,427	(27)			117,979
Net interest income after provision for credit losses			579,395			84,087			59,207				722,689			372,111			157,261				1,252,061
Noninterest income
Service charges on deposit accounts			33,240			2,216			—				35,456			10,447			—				45,903
Loss on sale of securities			(40,989)			(68)			—				(41,057)			—			—				(41,057)
Other operating income			98,626			16,188			(4,215)	(19),(22)			110,599			56,631			—				167,230
Total noninterest income			90,877			18,336			(4,215)				104,998			67,078			—				172,076
Noninterest expenses
Salaries and benefits			236,682			36,356			—				273,038			160,192			—				433,230
Occupancy expenses			25,146			6,219			—				31,365			18,778			—				50,143
Technology and data processing			32,484			5,394			—				37,878			11,186			—				49,064
Amortization of intangible assets			8,781			1,069			14,640	(20)			24,490			5,223			39,938	(28)			69,651
Merger-related costs			—			2,577			—				2,577			—			145,421	(29)			147,998
Other expenses			127,278			16,435			(1,309)	(22)			142,404			79,675			—				222,079
Total noninterest expenses			430,371			68,050			13,331				511,752			275,054			185,359				972,165
Income before income taxes			239,901			34,373			41,661				315,935

	Page
DESCRIPTION OF THE SUPPORT AGREEMENTS		137
MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER		138
Tax Consequences of the Merger Generally		138
Cash Instead of Fractional Shares		139
Information Reporting and Backup Withholding		139
DESCRIPTION OF ATLANTIC UNION CAPITAL STOCK		140
Atlantic Union Common Stock		140
Limitation of Liability and Indemnification of Officers and Directors		142
Transfer Agent and Registrar		144
Listing		144
COMPARISON OF SHAREHOLDERS’ RIGHTS		145
LEGAL MATTERS		160
EXPERTS		161
DEADLINES FOR SUBMITTING SHAREHOLDER PROPOSALS		162
Atlantic Union		162
Sandy Spring		162
WHERE YOU CAN FIND MORE INFORMATION		164
Annex A Agreement and Plan of Merger, dated as of October 21, 2024, by and between Sandy Spring Bancorp, Inc. and Atlantic Union Bankshares Corporation.		A-1
Annex B Support Agreement, dated as of October 21, 2024, by and between Sandy Spring Bancorp, Inc. and each of the shareholders of Atlantic Union Bankshares Corporation listed on the signature pages therein		B-1
Annex C Support Agreement, dated as of October 21, 2024, by and between Atlantic Union Bankshares Corporation and each of the stockholders of Sandy Spring Bancorp, Inc. listed on the signature pages therein		C-1
Annex D Opinion of Morgan Stanley & Co. LLC.		D-1
Annex E Opinion of Keefe, Bruyette & Woods, Inc.		E-1

	Page
THE ATLANTIC UNION SPECIAL MEETING		50
Date, Time and Place of the Meeting		50
Matters to Be Considered		50
Recommendation of the Atlantic Union Board of Directors		50
Record Date and Quorum		50
Broker Non-Votes		51
Vote Required; Treatment of Abstentions; Broker Non-Votes and Failure to Vote		51
Attending the Special Meeting		51
Proxies		52
Shares Held in Street Name		53
Revocability of Proxies		53
Delivery of Proxy Materials		53
Solicitation of Proxies		54
Assistance		54
ATLANTIC UNION PROPOSALS		55
Proposal 1: Atlantic Union Merger Proposal		55
Proposal 2: Atlantic Union Adjournment Proposal		55
THE SANDY SPRING SPECIAL MEETING		57
SANDY SPRING PROPOSALS		62
INFORMATION ABOUT ATLANTIC UNION		64
INFORMATION ABOUT SANDY SPRING		65
THE MERGER		66
Terms of the Merger		66
Background of the Merger		66
Atlantic Union’s Reasons for the Merger; Recommendation of the Atlantic Union Board of Directors		74
Opinion of Atlantic Union’s Financial Advisor		76
Sandy Spring’s Reasons for the Merger; Recommendation of the Sandy Spring Board of Directors		87
Opinion of Sandy Spring’s Financial Advisor		91
Certain Unaudited Prospective Financial Information		102
Consensus “Street Forecasts” used by Atlantic Union		103
Interests of Atlantic Union’s Directors and Executive Officers in the Merger		106
Interests of Sandy Spring’s Directors and Executive Officers in the Merger		106
Governance of Atlantic Union After the Merger		111
Accounting Treatment		111
Regulatory Approvals		111
Stock Exchange Listings		113
Appraisal or Dissenters’ Rights in the Merger		114
THE TRANSACTION AGREEMENTS		115
Description of the Merger Agreement		115