Registration of Securities issued in Business Combination Transaction Form S 4

As filed with the Securities and Exchange Commission on November 15, 2024

Registration No. 333-      

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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

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FORM S-4

REGISTRATION STATEMENT

UNDER

THE SECURITIES ACT OF 1933

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BYLINE BANCORP, INC.

(Exact name of registrant as specified in its charter)

180 North LaSalle Street, Suite 300

Chicago, Illinois 60601

(773) 244-7000

(Address, including zip code, and telephone number, including area code, of Registrant's principal executive offices)

Alberto J. Paracchini

President

Byline Bancorp, Inc.

180 North LaSalle Street, Suite 300

Chicago, Illinois 60601

(773) 244-7000

(Name, address, including zip code, and telephone number, including area code, of agent for service)

Copies to:

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this registration statement becomes effective and upon completion of the merger.

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.

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:

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.

fully diluted basis was approximately $[ ] million. We urge you to obtain current market quotations for shares of Byline common stock.

We cannot complete the merger unless we obtain the necessary approval from the stockholders of First Security as described in the attached proxy statement/prospectus. Accordingly, First Security will hold a special meeting of its stockholders in connection with the merger at [ ] [a.m./p.m.], Central Time, on [ ], [ ], virtually only at [ ] (which we refer to as the special meeting). First Security stockholders will be asked to vote to adopt the merger agreement (which we refer to as the merger proposal), and to approve one or more adjournments of the special meeting, if necessary or appropriate, as determined by First Security, including adjournments to permit further solicitation of proxies in favor of the merger proposal (which we refer to as the adjournment proposal). Approval of the merger proposal requires the affirmative vote of holders of a majority of the outstanding shares of First Security common stock entitled to vote at the special meeting. The adjournment proposal will be approved if a majority of the shares of First Security common stock duly voted on the adjournment proposal at the special meeting are voted in favor of the adjournment proposal.

First Security's board of directors has unanimously approved the merger agreement, has unanimously determined that the merger agreement and the transactions contemplated thereby, including the merger, are advisable and in the best interests of First Security and its stockholders, and unanimously recommends that First Security stockholders vote "FOR" the merger proposal and "FOR" the adjournment proposal.

YOUR VOTE IS VERY IMPORTANT. We cannot complete the merger unless First Security's stockholders approve the merger proposal.

Regardless of whether you plan to attend the virtual special meeting, please submit a proxy to have your shares voted as promptly as possible by signing and returning the accompanying proxy card in the provided postage-paid return envelope. You may also cast your vote live at the virtual special meeting.

The attached proxy statement/prospectus contains a more complete description of the merger agreement, the merger and the special meeting. You should read the entire proxy statement/prospectus carefully because it contains important information about the merger agreement, the merger and the special meeting. In particular, you should read the information under the section entitled "Risk Factors" beginning on page 26. You may also obtain information about Byline from documents that it has filed with the Securities and Exchange Commission (which we refer to as the SEC).

Thank you for your cooperation and continued support.

Sincerely,
Daniel R. Wirtz
Chairman and President
First Security Bancorp, Inc.

Neither the SEC nor any state securities regulatory body has approved or disapproved of the securities to be issued under this proxy statement/prospectus or determined if this proxy statement/prospectus is truthful 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 nonbank subsidiary of any of the parties, and they are not insured by the Federal Deposit Insurance Corporation or any other governmental agency.

This proxy statement/prospectus is dated [ ], 2024, and is first being mailed to First Security's stockholders on or about [ ], 2024.

VP/#68475597.6

NOTICE OF SPECIAL MEETING OF STOCKHOLDERS TO BE HELD ON [ ]

Dear Fellow Stockholders of First Security Bancorp, Inc.:

First Security Bancorp, Inc., a Delaware corporation (which we refer to as First Security), will hold a special meeting of First Security stockholders, at [ ] [a.m./p.m.], Central Time, on [ ], [ ], virtually only at [ ] (which we refer to as the special meeting), to consider and vote on the following matters:

1. a proposal (which we refer to as the merger proposal) to adopt the Agreement and Plan of Merger, dated as of September 30, 2024, by and between First Security and Byline Bancorp, Inc., a Delaware corporation (which we refer to as Byline), as such agreement may be amended from time to time (which we refer to as the merger agreement and a copy of which is attached to the accompanying proxy statement/prospectus as Appendix A), pursuant to which First Security will be merged with and into Byline, with Byline as the surviving corporation (which we refer to as the merger); and

2. a proposal (which we refer to as the adjournment proposal) to adjourn the special meeting, if necessary or appropriate, as determined by First Security, to solicit additional proxies in favor of the merger proposal.

We have fixed the close of business on [ ], as the record date for the special meeting. Only First Security stockholders of record on that date are entitled to notice of, and to vote at, the special meeting or any adjournment or postponement of the special meeting. Approval of the merger proposal requires the affirmative vote of a majority of the outstanding shares of First Security common stock entitled to vote at the special meeting. The adjournment proposal will be approved if a majority of the shares of First Security common stock duly voted on the adjournment proposal at the special meeting are voted in favor of the adjournment proposal.

First Security's board of directors has unanimously approved the merger agreement, has unanimously determined that the merger agreement and the transactions contemplated thereby, including the merger, are advisable and in the best interests of First Security and its stockholders, and unanimously recommends that First Security stockholders vote "FOR" the merger proposal and "FOR" the adjournment proposal.

Your vote is very important. We cannot complete the merger unless First Security's stockholders approve the merger proposal. Regardless of whether you plan to attend the virtual special meeting, please submit a proxy to have your shares voted as promptly as possible by signing and returning the accompanying proxy card in the provided postage-paid return envelope. You may also cast your vote live at the virtual special meeting.

First Security stockholders should not send in any stock certificate(s) with their proxy card. If the merger proposal is approved and the merger closes, transmittal materials with instructions for the submission of First Security stock certificates will be provided to First Security stockholders under separate cover and the stock certificates should be sent at that time.

Please note that, under Section 262 of the Delaware General Corporation Law, as amended (which we refer to as the DGCL), a copy of which is attached as Appendix B to the accompanying proxy statement/prospectus, holders of First Security common stock who do not vote in favor of the merger proposal will have the right to seek appraisal of the fair value of their shares of First Security common stock as determined under the DGCL if the merger is completed, but only if they submit a written demand for such an appraisal prior to the vote on the merger proposal and comply with the other Delaware law procedures explained in the accompanying proxy statement/prospectus. Holders of First Security common stock who do not vote in favor of the merger proposal and who properly submit a written demand for such an appraisal prior to the vote on the merger proposal and comply with the other DGCL procedures will not receive the merger consideration, but instead will have their First Security common stock converted into the right to receive payment of such appraised value in accordance with the DGCL procedures as explained in the accompanying proxy statement/prospectus.

The accompanying proxy statement/prospectus provides a detailed description of the special meeting, the merger, the documents related to the merger, and other related matters. We urge you to read the proxy statement/prospectus, including any documents incorporated in the proxy statement/prospectus by reference, and its appendices carefully and in their entirety.

If you have any questions concerning the merger, the merger agreement or the proxy statement/prospectus, would like additional copies of the proxy statement/prospectus without charge or need help voting your shares of First Security common stock, please contact Wade Wacholz at (312) 475-6419.

WHERE YOU CAN FIND MORE INFORMATION

Byline Bancorp, Inc.

Byline Bancorp, Inc. (which we refer to as Byline) files annual, quarterly and special reports, proxy statements and other business and financial information with the Securities and Exchange Commission (which we refer to as the SEC). Byline files reports and other business and financial information with the SEC electronically, and the SEC maintains a website located at http://www.sec.gov containing this information. You may also obtain these documents, free of charge, from Byline at its investor relations website, http://www.bylinebancorp.com under the tab "Financial Information" and then under "SEC Filings."

Byline has filed a registration statement on Form S-4, of which this proxy statement/prospectus forms a part. As permitted by SEC rules, this document does not contain all of the information included in the registration statement or in the exhibits or schedules to the registration statement. You may read and copy the registration statement, including any amendments, schedules and exhibits, at the address set forth below. Statements contained in this document as to the contents of any contract or other documents referred to in this document are not necessarily complete. In each case, you should refer to the copy of the applicable contract or other document filed as an exhibit to the registration statement. This document incorporates by reference documents that Byline has previously filed with the SEC. They contain important information about Byline and its financial condition. For more information, please see the section entitled "Incorporation of Certain Documents by Reference." These documents are available without charge to you upon written or oral request to Byline's principal executive offices. The address and telephone number of Byline's principal executive office is listed below:

Byline Bancorp, Inc.

Attn: Brooks Rennie, Investor Relations Director

180 North LaSalle Street, Suite 300

Chicago, Illinois 60601

(773) 244-7000

Byline's common stock is traded on the New York Stock Exchange under the symbol "BY".

First Security Bancorp, Inc.

First Security Bancorp, Inc. (which we refer to as First Security) does not have a class of securities registered under Section 12 of the Securities Exchange Act of 1934, as amended (which we refer to as the Exchange Act), is not subject to the reporting requirements of Section 13(a) or 15(d) of the Exchange Act and, accordingly, does not file documents and reports with the SEC.

If you are a First Security stockholder and have any questions concerning the merger proposal, the merger agreement or the proxy statement/prospectus, would like additional copies of the proxy statement prospectus without charge or need help voting your shares of First Security common stock, please contact Wade Wacholz at (312) 475-6419 or at the following address:

First Security Bancorp, Inc.

Attn: Wade Wacholz

7315 W. Grand Avenue

Elmwood Park, IL 60707

(312) 475-6419

To obtain timely delivery of these documents, you must request the information no later than [ ], in order to receive them before First Security's special meeting of stockholders.

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QUESTIONS AND ANSWERS

The following are some questions that you may have regarding the proposals being considered at the virtual special meeting of stockholders of First Security (which we refer to as the special meeting). You should carefully read the remainder of this proxy statement/prospectus because the information in this section may not provide all the information that might be important to you in determining how to vote. Additional important information is also contained in the appendices to, and the documents incorporated by reference in, this proxy statement/prospectus. See "Incorporation of Certain Documents by Reference" beginning on page 108.

In order to complete the merger, First Security's stockholders must approve the merger agreement. First Security stockholders are also being asked to approve one or more adjournments of the special meeting, if necessary or appropriate, as determined by First Security, including adjournments to permit further solicitation of proxies in favor of the merger agreement. For additional information regarding the proposals to be presented to First Security stockholders, refer to the section entitled "First Security Proposals." First Security will hold a virtual special meeting to consider and vote on these proposals. This proxy statement/prospectus contains important information about the merger and the special meeting, and you should read it carefully and in its entirety. The accompanying voting materials allow you to vote your shares of First Security common stock, par value $0.01 per share (which we refer to as First Security common stock), without attending the special meeting in person.

We encourage you to submit a proxy to vote your shares of First Security common stock as promptly as possible so that your shares may be represented and voted at the special meeting.

This proxy statement/prospectus is also a prospectus with respect to the offering of shares of Byline common stock, par value $0.01 per share (which we refer to as Byline common stock) to be issued in connection with the merger.

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Based on the number of outstanding shares of First Security common stock as of the date of this proxy statement/prospectus, we expect that the exchange ratio will be equal to 2.1794.

First Security stockholders also will receive cash in lieu of any fractional shares of Byline common stock that would otherwise be issuable in connection with the merger. For more information on the potential adjustments to the exchange ratio and cash in lieu of fractional shares, see "Description of the Merger Agreement - Merger Consideration."

In the event that First Security realizes certain recoveries on identified loans sold by it or First Security Bank prior to closing, provided that First Security's closing tangible common equity is equal to or greater than the minimum required as set forth in the merger agreement, then the aggregate merger consideration will be increased as set forth in the merger agreement. For more information on the potential adjustments to the merger consideration, see "Description of the Merger Agreement - Merger Consideration."

Based on the closing price of Byline common stock as reported on the NYSE of $26.16 as of September 27, 2024, the trading day prior to the day on which the public announcement of the merger was made, and assuming there are no adjustments to the exchange ratio pursuant to the merger agreement, the implied value of the merger consideration was approximately $57.01 per share of First Security common stock and the implied aggregate transaction value on a fully diluted basis was approximately $38.4 million.

2

Based on the closing price of Byline common stock as reported on the NYSE of $[ ] as of [ ], 2024, the last practicable date before the date of this proxy statement/prospectus, and assuming there are no adjustments to the exchange ratio pursuant to the merger agreement, the implied value of the merger consideration was approximately $[ ] per share of First Security common stock and the implied aggregate transaction value on a fully diluted basis was approximately $[ ] million. We urge you to obtain current market quotations for shares of Byline common stock.

First Security stockholder approval is required to complete the merger. First Security does not intend to transact any business at the special meeting other than as listed above.

3

The Adjournment Proposal: The adjournment proposal requires the affirmative vote of the majority of the shares of First Security common stock duly voted on the adjournment proposal at the special meeting.

You may also cast your vote live at the virtual special meeting.

4

Abstentions will be counted as represented at the special meeting for purposes of determining the presence or absence of a quorum for all matters to be voted on at the special meeting.

For the merger proposal, if a First Security stockholder present in person at the special meeting abstains from voting, or responds by proxy with an "abstain" vote, it will have the same effect as a vote cast "AGAINST" the proposal. If a First Security stockholder is not present in person at the special meeting and does not respond by proxy, it will also have the same effect as a vote cast "AGAINST" the proposal.

The adjournment proposal requires the affirmative vote of the majority of the shares of First Security common stock duly voted on the adjournment proposal at the special meeting. Accordingly, for purposes of the adjournment proposal, abstentions will not affect the outcome, as they will not be counted as shares duly voted on the adjournment proposal at the special meeting.

If you are a First Security stockholder of record as of the record date and you choose to send a written notice of revocation or to mail a new proxy card, you must submit your notice of revocation or your new proxy to:

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First Security Bancorp, Inc.

Attn: Wade Wacholz
7315 W. Grand Avenue
Elmwood Park, IL 60707
(312) 475-6419

Any proxy that you submitted may also be revoked by voting live at the virtual special meeting.

For a more detailed discussion of the material U.S. federal income tax consequences of the merger, please see the section entitled "Material U.S. Federal Income Tax Consequences of the Merger."

The tax consequences of the merger to any particular stockholder will depend on that stockholder's particular facts and circumstances. Accordingly, you are urged to consult your tax advisor to determine your tax consequences from the merger.

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SUMMARY

This summary highlights selected information included in this proxy statement/prospectus and does not contain all of the information that may be important to you. You should read this entire proxy statement/prospectus and its appendices and the other documents to which the parties refer before you decide how to vote with respect to the proposals. In addition, Byline incorporates by reference important business and financial information about Byline into this proxy statement/prospectus that is not included in or delivered with this proxy statement/prospectus. For a description of this information, please see the section entitled "Incorporation of Certain Documents by Reference." You may obtain the information Byline has incorporated by reference into this proxy statement/prospectus without charge by following the instructions in the section entitled "Where You Can Find More Information" in the forepart of this proxy statement/prospectus. Each item in this summary includes a page reference directing you to a more complete description of that item.

The Merger and the Merger Agreement (pages 38 and 75)

The terms and conditions of the merger are contained in the merger agreement, a copy of which is attached to this proxy statement/prospectus as Appendix A. The parties encourage you to read the merger agreement carefully, as it is the legal document that governs the merger.

Under the terms of the merger agreement, First Security will merge with and into Byline, with Byline as the surviving corporation. Immediately following the merger, First Security Bank will merge with and into Byline Bank, with Byline Bank as the resulting bank.

Merger Consideration (page 76)

In the merger, each share of First Security common stock outstanding immediately prior to the effective time, other than shares, if any, owned by First Security or Byline (other than shares held in a trust, fiduciary, or nominee capacity or as a result of debts previously contracted), or as to which statutory appraisal rights have been properly exercised and perfected (for more information on appraisal rights, see "The Merger - Appraisal Rights"), will be converted into the right to receive a number of shares of Byline common stock equal to the exchange ratio, subject to adjustment as set forth in the merger agreement and as further described in the section entitled "Description of the Merger Agreement - Merger Consideration." Based on the number of outstanding shares of First Security common stock as of the date of this proxy statement/prospectus, we expect that the exchange ratio will be equal to 2.1794. For each fractional share of Byline common stock that would otherwise be issued, Byline will pay cash in an amount equal to the fraction of a share (rounded to the nearest cent) of Byline common stock which the holder would otherwise be entitled to receive multiplied by the Byline closing price. No interest will be paid or accrue on the cash payable to holders in lieu of fractional shares.

Recommendation of the First Security Board of Directors (page 33)

After careful consideration, the First Security board unanimously recommends that First Security stockholders vote "FOR" the merger proposal and "FOR" the adjournment proposal. For a more complete description of First Security's reasons for the merger and the recommendations of the First Security board, please see the section entitled "The Merger - Recommendation of the First Security Board of Directors and First Security's Reasons for the Merger."

Byline's Reasons for the Merger (page 64)

The Byline board consulted with Byline senior management, as well as its financial and legal advisors, and considered a number of factors and risks in evaluating the merger agreement. The Byline board considered all these factors as a whole and overall considered the factors to be favorable to, and to support its determination to approve, entering into the merger agreement. For a more complete description of Byline's reasons for the merger and the recommendations of the Byline board, please see the section entitled "The Merger - Byline's Reasons for the Merger."

Opinion of First Security's Financial Advisor (page 46)

At the September 30, 2024 meeting of the First Security board, a representative of First Security's financial advisor, D.A. Davidson & Co. (which we refer to as D.A. Davidson), rendered D.A. Davidson's oral opinion, which was subsequently confirmed by delivery of a written opinion to the First Security board, dated September 30, 2024, as to the fairness, as of such date, from a financial point of view, to the holders of First Security's outstanding common stock of the exchange ratio to be received by such holders in the merger pursuant to the merger agreement, based upon and subject to the qualifications, assumptions and other matters considered in connection with the preparation of its opinion.

The full text of the written opinion of D.A. Davidson, dated September 30, 2024, which sets forth, among other things, the various qualifications, assumptions and limitations on the scope of the review undertaken by D.A. Davidson, is attached as Appendix C to this proxy statement/prospectus. D.A. Davidson provided its opinion for the information and assistance of the First Security board (solely in its capacity as such) in connection with, and for purposes of, the First Security board's consideration of the merger and D.A. Davidson's opinion only addressed whether the exchange ratio to be received by the holders of First Security common stock in the merger pursuant to the merger agreement was fair, from a financial point of view, to such holders. The opinion of D.A. Davidson does not address any other term or aspect of the merger agreement or the transactions contemplated thereby. D.A. Davidson's opinion does not constitute a recommendation to the First Security board or any holder of First Security common stock as to how the First Security board, such stockholder or any other person should vote or otherwise act with respect to the merger or any other matter.

First Security Special Meeting of Stockholders (page 33)

The special meeting will be held at [ ] [a.m./p.m.], Central Time, on [ ], [ ], virtually only at [ ]. At the special meeting, holders of shares of First Security common stock will be asked to approve the merger proposal and the adjournment proposal.

The First Security board has fixed the close of business on [ ], as the record date for determining the holders of shares of First Security common stock entitled to receive notice of and to vote at the special meeting. As of the close of business on the record date, there were [674,012] shares of First Security common stock outstanding and entitled to vote at the special meeting held by approximately [31] stockholders of record. Each share of First Security common stock entitles the holder to one vote on each proposal to be considered at the special meeting.

As of the close of business on the record date, directors and executive officers of First Security and their affiliates were entitled to vote [303,811] shares of First Security common stock, representing approximately [45.07]% of the shares of First Security common stock outstanding. As of the close of business on the record date, Byline beneficially held [no] shares of First Security common stock.

On September 30, 2024, Arthur Wirtz III, a director of First Security, and Daniel R. Wirtz, First Security's Chairman and President, entered into a voting agreement with Byline solely in their capacities as record and beneficial holders of First Security common stock. Under this agreement, Mr. Arthur Wirtz and Mr. Daniel Wirtz have agreed, subject to certain exceptions, to vote their shares of First Security common stock: (a) in favor of the merger agreement and the transactions contemplated by the merger agreement, including, without

limitation, the merger; (b) against any action or agreement which would result in a breach of any term of, or any other obligation of First Security under, the merger agreement or which is reasonably likely to result in any conditions to Byline's obligations under the merger agreement not being fulfilled; and (c) against any action or agreement which would impede or materially delay or interfere with the transactions contemplated by the merger agreement. The [303,811] shares of First Security common stock subject to the voting agreement represented approximately [45.07]% of First Security's outstanding shares of common stock as of [ ], the record date for the special meeting. The voting obligations under the voting agreement will automatically terminate upon the earlier of (a) the effective time and (b) the date of termination of the merger agreement in accordance with its terms.

Approval of the merger proposal requires the affirmative vote of a majority of the outstanding shares of First Security common stock entitled to vote at the special meeting. Approval of the adjournment proposal requires the affirmative vote of the majority of the shares of First Security common stock duly voted on the adjournment proposal at the special meeting.

Interests of First Security Directors and Executive Officers in the Merger (page 65)

In considering the recommendation of the First Security board, First Security stockholders should be aware that the directors and executive officers of First Security have certain interests in the merger that may be different from, or in addition to, the interests of First Security stockholders generally. The First Security board was aware of these interests and considered them, among other matters, in making its recommendation that First Security stockholders vote to approve the merger proposal. These interests include:

For a more complete description of the interests of First Security's directors and executive officers in the merger, see "The Merger - Interests of First Security Directors and Executive Officers in the Merger."

Management and Board of Directors of Byline After the Merger (page 65)

The directors and officers of Byline immediately prior to the effective time will be the directors and officers of the surviving corporation after the merger until the earlier of their resignation or removal or until their respective successors are duly appointed and qualified.

Regulatory Approvals Required for the Merger (page 66)

The merger cannot proceed without obtaining all requisite regulatory approvals. Byline and First Security have agreed to use their reasonable best efforts to obtain the required approvals. The merger of Byline and First Security is subject to prior approval of the Board of Governors of the Federal Reserve System (which we refer to as the Federal Reserve) or through delegated authority to the Federal Reserve Bank of Chicago, unless the Federal Reserve waives this requirement pursuant to Regulation Y under the BHC Act. In accordance with Regulation Y, on November 4, 2024, Byline submitted a request to the Federal Reserve Bank of Chicago for a waiver from the requirement under Section 3 of the BHC Act for prior approval of the Federal Reserve.

Immediately following the completion of the merger, First Security Bank will merge with and into Byline Bank, with Byline Bank as the resulting bank. The bank merger is subject to approval by the Federal Deposit Insurance Corporation (which we refer to as the FDIC) and the Illinois Department of Financial and Professional Regulation (which we refer to as the IDFPR). Byline Bank submitted applications with the FDIC and the IDFPR on November 4, 2024 seeking the necessary approvals for the bank merger.

The U.S. Department of Justice has authority to comment on the merger and the bank merger during the regulatory approval process of the FDIC, and will have 15 days following the approval by the FDIC on [ ], 2024 to challenge the merger on antitrust grounds.

Neither Byline nor First Security is aware of any material governmental approvals or actions that are required for completion of the merger or bank merger other than those described above. It is presently contemplated that if any such additional governmental approvals or actions are required, those approvals or actions will be sought. However, there can be no assurance that any additional approvals or actions will be obtained.

Conditions to Consummation of the Merger (page 88)

The respective obligation of each party to consummate the merger is subject to the fulfillment or written waiver at or prior to the closing of each of the following conditions:

First Security's obligation to consummate the merger is also subject to the fulfillment or written waiver of each of the following conditions:

Byline's obligation to consummate the merger is also subject to the fulfillment or written waiver of each of the following conditions:

Acquisition Proposals (page 87)

Under the terms of the merger agreement, First Security has agreed that it will not and will cause its subsidiaries and affiliates not to:

However, the above restriction does not prevent First Security or its board of directors from:

only if, however, in each case referred to in the bullet points above, (i) the First Security board concludes in good faith (after consultation with outside legal counsel and financial advisor) that (A) such acquisition proposal either constitutes a superior proposal (as defined in the section entitled "Description of the Merger Agreement - Acquisition Proposals") or would reasonably be expected to result in a superior proposal and (B) the failure to

take such action would reasonably be expected to violate the directors' fiduciary duties under applicable law; and (ii) First Security has provided notice to Byline of its intention to provide information to the person who has made such acquisition proposal, and First Security has provided such information to Byline.

Further, the merger agreement provides that, unless the merger agreement is contemporaneously terminated in accordance with its terms, the First Security board will not cause or permit First Security to enter into any letter of intent, memorandum of understanding, agreement in principle, acquisition agreement, merger agreement or other agreement (other than a confidentiality agreement referred to above) relating to any acquisition proposal.

For more information, please see the section entitled "Description of the Merger Agreement - Acquisition Proposals."

Termination of the Merger Agreement (page 90)

Byline and First Security may mutually agree in writing to terminate the merger agreement at any time prior to the effective time. Subject to certain conditions described in the merger agreement, either Byline or First Security may also terminate the merger agreement if:

In addition, First Security may terminate the merger agreement if there is a breach of any of the covenants, agreements, representations or warranties of Byline such that the applicable conditions to First Security's obligation to close the merger set forth in the merger agreement would not be satisfied, and such breach has not been, or cannot be, cured prior to the earlier of the outside date or 30 days after notice to Byline from First Security.

In addition, Byline may terminate the merger agreement if there is a breach of any of the covenants, agreements, representations or warranties of First Security such that the applicable conditions to Byline's obligation to close the merger set forth in the merger agreement would not be satisfied, and such breach has not been, or cannot be, cured prior to the earlier of the outside date or 30 days after notice to First Security from Byline.

Byline may also terminate the merger agreement prior to the adoption of the merger agreement by the First Security stockholders, if:

For more information, please see the section entitled "Description of the Merger Agreement - Termination of the Merger Agreement."

Termination Fee (page 91)

First Security has agreed to pay to Byline a cash termination fee in an amount equal to $1,653,000 in the following circumstances:

For more information, please see the section entitled "Description of the Merger Agreement - Termination Fee."

Voting Agreement (page 91)

Arthur Wirtz III, a director of First Security, and Daniel R. Wirtz, First Security's Chairman and President, entered into a voting agreement with Byline solely in their capacities as record and beneficial holders of First Security common stock. Under this agreement, Mr. Arthur Wirtz and Mr. Daniel Wirtz have agreed, subject to certain exceptions, to vote their shares of First Security common stock: (a) in favor of the merger agreement and the transactions contemplated by the merger agreement, including, without limitation, the merger; (b) against any action or agreement which would result in a breach of any term of, or any other obligation of First Security under, the merger agreement or which is reasonably likely to result in any conditions to Byline's obligations

under the merger agreement not being fulfilled; and (c) against any action or agreement which would impede or materially delay or interfere with the transactions contemplated by the merger agreement. The [303,811] shares of First Security common stock subject to the voting agreement represented approximately [45.07]% of First Security's outstanding shares of common stock as of [ ], the record date for the special meeting. The voting obligations under the voting agreement will automatically terminate upon the earlier of (a) the effective time and (b) the date of termination of the merger agreement in accordance with its terms.

For more information, please see the section entitled "Description of the Merger Agreement - Voting Agreement."

Accounting Treatment of the Merger (page 68)

For accounting and financial reporting purposes, the merger will be accounted for under the acquisition method of accounting for business combinations in accordance with accounting principles generally accepted in the United States of America (which we refer to as GAAP).

Material U.S. Federal Income Tax Consequences of the Merger (page 92)

The merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Code. However, neither First Security nor Byline has requested or received a ruling from the IRS that the merger will qualify as a reorganization. If the merger qualifies as a reorganization for U.S. federal income tax purposes, a U.S. holder of First Security common stock who receives Byline common stock in exchange for shares of First Security common stock pursuant to the merger, generally will recognize gain (but not loss) in an amount equal to the amount of gain realized (i.e., the excess of the amount of the fair market value of the Byline common stock received pursuant to the merger over such holder's adjusted tax basis in the shares of First Security common stock surrendered in the exchange). In addition, a U.S. holder of First Security common stock generally will recognize gain or loss with respect to cash received in lieu of a fractional share of Byline common stock. It is a condition to the completion of the merger that Byline and First Security receive written opinions from their respective counsel to the effect that the merger will qualify as a reorganization within the meaning of Section 368(a) of the Code.

Tax matters are complicated and the tax consequences of the merger to each First Security stockholder may depend on such stockholder's particular facts and circumstances. First Security stockholders are urged to consult their tax advisors to understand fully the tax consequences to them of the merger. For more information, please see the section entitled "Material U.S. Federal Income Tax Consequences of the Merger."

Comparison of Stockholders' Rights (page 95)

The rights of First Security stockholders who become Byline stockholders after the merger will continue to be governed by the DGCL. However, their rights will be governed by Byline's amended and restated certificate of incorporation (which we refer to as Byline's certificate of incorporation) and Byline's amended and restated bylaws (which we refer to as Byline's bylaws), rather than by the certificate of incorporation and bylaws of First Security. For more information, please see the section entitled "Comparison of Stockholders' Rights."

The Parties (page 36)

Byline Bancorp, Inc.

180 North LaSalle Street, Suite 300

Chicago, Illinois 60601

Telephone: (773) 244-7000

Byline is a bank holding company incorporated in the State of Delaware and headquartered in Chicago, Illinois. Byline's wholly-owned banking subsidiary, Byline Bank, an Illinois state-chartered bank, is a full service commercial bank, and has been a part of the Chicago banking community for over 100 years. Byline Bank offers a broad range of banking products and services to small and medium sized businesses, commercial real estate and

financial sponsors and to consumers who generally live or work near its branches. Byline also offers online account opening to consumer and business customers through its website and provides trust and wealth management services to its customers. In addition to its traditional commercial banking business, Byline provides small ticket equipment leasing solutions through Byline Financial Group, a wholly-owned subsidiary of Byline Bank, headquartered in Bannockburn, Illinois, with sales offices in Illinois, and sales representatives in Illinois, Michigan, New Jersey, and New York. Byline also participates in U.S. government guaranteed lending programs and originate U.S. government guaranteed loans. Byline Bank is a leading originator of Small Business Administration (which we refer to as SBA) loans in the country and the most active 7(a) and 504 lender in Illinois for the fiscal year ended September 30, 2024.

As of September 30, 2024, Byline had consolidated total assets of $9.4 billion, total gross loans and leases outstanding of $6.9 billion, total deposits of $7.5 billion, and total stockholders' equity of $1.1 billion.

Byline common stock is traded on the NYSE under the ticker symbol "BY".

First Security Bancorp, Inc.

7315 W. Grand Avenue

Elmwood Park, IL 60707

Telephone: (708) 453-3131

First Security is a one bank holding company headquartered in Elmwood Park, Illinois founded in 1993. First Security is incorporated under the laws of the State of Delaware. First Security is engaged in the banking business through its wholly-owned banking subsidiary, First Security Trust and Savings Bank, an Illinois state-chartered bank that offers commercial and retail banking services at its sole office in Elmwood Park, Illinois. First Security Bank is principally engaged in the business of attracting deposits from the general public and using such deposits, together with borrowings and other funds, to originate commercial, consumer and mortgage loans.

As of September 30, 2024, First Security Bank had total assets of approximately $361.2 million, total loans of approximately $199.7 million and total deposits of approximately $325.5 million.

Risk Factors (page 26)

Before voting at the special meeting, you should carefully consider all of the information contained in or incorporated by reference into this proxy statement/prospectus, including the risk factors set forth in the section entitled "Risk Factors" or described in Byline's Annual Report on Form 10-K for the year ended December 31, 2023, and other reports filed with the SEC, which are incorporated by reference into this proxy statement/prospectus. Please see "Where You Can Find More Information" and "Incorporation of Certain Documents by Reference."

SELECTED HISTORICAL CONSOLIDATED FINANCIAL INFORMATION FOR BYLINE

The following table summarizes certain selected historical consolidated financial data of Byline for the periods and as of the dates indicated. You should read this in conjunction with Byline's consolidated financial statements and the notes to the consolidated financial statements contained in reports that Byline has previously filed with the SEC. Historical financial information for Byline can be found in its Annual Report on Form 10-K for the year ended December 31, 2023 and in its quarterly reports on Form 10-Q for the quarters ended March 31, 2024, June 30, 2024 and September 30, 2024. Please see the section entitled "Where You Can Find More Information" for instructions on how to obtain the information that has been incorporated by reference. You should not assume the results of operations for past periods indicate results for any future period. Byline's management uses the non-GAAP financial measures set forth herein in its analysis of Byline's performance. Byline believes that these non-GAAP financial measures provide useful information to management and investors, however, you should not view these disclosures as a substitute for results determined in accordance with GAAP financial measures.

GAAP RECONCILIATION AND MANAGEMENT
EXPLANATION OF NON-GAAP FINANCIAL MEASURES

Some of the financial measures included in "Selected Historical Consolidated Financial Information for Byline" are not measures of financial performance in accordance with GAAP. Byline's management uses the non-GAAP financial measures set forth below in its analysis of Byline's performance:

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Byline believes that these non-GAAP financial measures provide useful information to its management and investors that is supplementary to its financial condition, results of operations and cash flows computed in accordance with GAAP; however, Byline acknowledges that its non-GAAP financial measures have a number of limitations. As such, you should not view these disclosures as a substitute for results determined in accordance with GAAP financial measures that Byline and other companies use. Byline's management also uses these measures for peer comparison.

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The following reconciliation tables provide a more detailed analysis of the non-GAAP financial measures discussed herein:

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COMPARATIVE MARKET INFORMATION

Byline Information

Byline common stock is traded on the NYSE under the symbol "BY".

The following table sets forth the closing sale price per share of Byline common stock on September 27, 2024, the trading day prior to the day on which the public announcement of the signing of the merger agreement was made, and on [ ], 2024, the latest practicable date before the date of this proxy statement/ prospectus. The following table also includes the equivalent market value per share of First Security common stock on September 27, 2024 and [ ], 2024, based on 674,012 shares of First Security common stock outstanding as of such dates, determined with an exchange ratio of 2.1794.

First Security Information

There is no established public trading market for shares of First Security common stock and no broker makes a market in the stock.

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RISK FACTORS

In addition to the other information contained in or incorporated by reference into this proxy statement/ prospectus, including the matters addressed under the caption entitled "Cautionary Statement Regarding Forward-Looking Statements," and in "Item 1A. Risk Factors" in Byline's Annual Report on Form 10-K for the year ended December 31, 2023, First Security stockholders should carefully consider the following factors in deciding whether to vote for First Security's proposals. Please see the sections entitled "Where You Can Find More Information" and "Incorporation of Certain Documents by Reference."

Risks Related to the Merger and Byline's and

First Security's Businesses upon Completion of the Merger

The value of the merger consideration will fluctuate based on the trading price of Byline common stock, therefore First Security stockholders cannot be certain of the market value of the merger consideration they will receive.

The number of shares of Byline common stock to be issued in the merger will not automatically adjust based on the trading price of Byline common stock, and the market value of those shares at the effective time may vary significantly from the current price of Byline common stock or the price of Byline common stock at the time of the special meeting. Accordingly, at the time of the special meeting, First Security stockholders will not know or be able to calculate the market value of the shares of Byline common stock they might receive upon the completion of the merger.

The market price of Byline common stock could be subject to significant fluctuations due to changes in sentiment in the market regarding Byline's operations or business prospects, including market sentiment regarding Byline's entry into the merger agreement. These risks may be affected by, among other things:

Stock price changes may also result from a variety of other factors, many of which are outside of the control of Byline and First Security, including changes in the business, operations or prospects of Byline or First Security, regulatory considerations, and general business, market, industry or economic conditions. For more information, see "Description of the Merger Agreement - Merger Consideration" and "Description of the Merger Agreement - Termination."

The merger consideration may be adjusted only under certain limited circumstances as set forth in the merger agreement. In the event that First Security realizes certain recoveries on identified loans sold by it or First Security Bank prior to closing, provided that First Security's closing tangible common equity is equal to or greater than the minimum required as set forth in the merger agreement, then the aggregate merger consideration will be increased as set forth in the merger agreement.

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The market price of Byline common stock after the merger may be affected by factors different from those affecting the shares of First Security or Byline currently.

Upon completion of the merger, holders of First Security common stock will become holders of Byline common stock. Byline's business differs from that of First Security. Accordingly, the results of operations of the combined company, and the market price of Byline common stock after the completion of the merger may be affected by factors different from those currently affecting the independent results of operations of each of Byline and First Security. For a discussion of the business and market of each of Byline and First Security and of some important factors to consider in connection with the business of each of Byline and First Security, please see "Information About the Companies."

First Security stockholders will have a reduced ownership and voting interest after the merger and will exercise less influence over management.

Upon the completion of the merger, each former First Security stockholder will have a percentage ownership of Byline that is smaller than such stockholder's current percentage ownership of First Security. Based on the number of issued and outstanding shares of Byline common stock and First Security common stock on [ ], 2024, and assuming no adjustment in the number of shares of Byline common stock to be issued as merger consideration pursuant to the merger agreement, stockholders of First Security, as a group, will receive shares in the merger constituting approximately [ ]% of Byline common stock expected to be outstanding immediately after the merger (without giving effect to any Byline common stock held by First Security stockholders prior to the merger). As a result, First Security stockholders, as a group, will have less influence on the board of directors, management and policies of Byline following the merger than they now have on the board of directors, management and policies of First Security.

Byline may fail to realize the anticipated benefits of the merger.

Byline and First Security have operated independently and will continue to do so until the completion of the merger. The success of the merger, including anticipated benefits and cost savings, will depend on, among other things, Byline's ability to successfully combine the businesses of Byline and First Security, including by minimizing any disruptions to the existing customer relationships and business functions of Byline or First Security, and avoiding any inconsistencies in standards, controls, procedures and policies. If Byline is not able successfully to achieve these objectives, the anticipated benefits of the merger may not be realized fully, or at all, or may take longer to realize than expected. Failure to achieve these anticipated benefits could result in increased costs, decreases in the amount of expected revenues and diversion of management's time and energy and could have an adverse effect on Byline's business, financial condition, operating results and prospects.

Among the factors considered by the boards of directors of each of Byline and First Security in connection with their respective approvals of the merger agreement were the anticipated benefits that could result from the merger. There can be no assurance that these benefits will be realized within the time periods contemplated or at all. To review the reasons for the merger in more detail, see "The Merger - Recommendation of the First Security Board of Directors and First Security's Reasons for the Merger."

Regulatory approvals may not be received, may take longer than expected or may impose conditions that are not presently anticipated or cannot be met.

Before the transactions contemplated in the merger agreement can be completed, various approvals must be obtained from bank regulatory agencies and other governmental authorities. In deciding whether to grant regulatory approval, the relevant governmental entities will consider a variety of factors, including the regulatory standing of each of the parties. An adverse condition or development in either party's regulatory standing or other factors could prevent or delay the receipt of one or more of the required regulatory approvals. Even if granted, the terms and conditions of the approvals may impose requirements, limitations or costs or place restrictions on the conduct of the combined company's business. Despite the parties' commitments to use their reasonable best efforts to obtain regulatory approvals, under the terms of the merger agreement, Byline and First Security will not

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be required to complete the merger if any such approval imposes a burdensome condition. There can be no assurance that regulators will not impose conditions, terms, obligations or restrictions and that such conditions, terms, obligations or restrictions will not have the effect of delaying the completion of the merger, imposing additional material costs on or materially limiting the revenues of the combined company following the merger or otherwise reduce the anticipated benefits of the merger if the merger were completed successfully within the expected timeframe. Additionally, the completion of the merger is subject to the satisfaction or waiver of certain other closing conditions, including the absence of certain orders, injunctions or decrees by any governmental authority that would prohibit or make illegal the completion of the merger. Please see the section entitled "Description of the Merger Agreement - Conditions to Consummation of the Merger."

Because of the closing conditions in the merger agreement and the ability of either Byline or First Security to terminate the merger agreement in specific instances, there can be no assurance when or if the merger will be completed.

The merger agreement is subject to a number of conditions that must be satisfied or waived to complete the merger. Those conditions include, among other things, (i) the accuracy of the other party's representations and warranties, subject to certain materiality standards, including the accuracy of the other party's representation and warranty of the absence of a material adverse effect on the other party since December 31, 2023, (ii) the other party's performance in all material respects of its obligations under the merger agreement, (iii) the adoption of the merger agreement and the transactions contemplated thereby by First Security stockholders, (iv) the absence of any proceeding in connection with, or that could prevent, delay, make illegal or interfere with, any of the transactions contemplated by the merger agreement, (v) the receipt of required regulatory approvals, including the approval of certain federal and state banking agencies, (vi) the effectiveness of the registration statement of which this proxy statement/prospectus forms a part, (vii) the receipt by each party of an opinion from such party's counsel to the effect that the merger qualifies as a "reorganization" within the meaning of Section 368(a) of the Code, and (viii) the approval for listing on the NYSE of the shares of Byline common stock issuable in the merger.

In addition, Byline's obligation to complete the merger is subject to appraisal rights having been exercised by the holders of no more than five percent of First Security common stock, receipt by First Security of certain third party consents and the delivery to Byline from First Security of the certificate or certificates representing the shares of common stock of First Security Bank held by First Security. These conditions to the closing of the merger may not be fulfilled in a timely manner or at all, and, accordingly, the merger may not be completed. In addition, the parties can mutually decide to terminate the merger agreement at any time, before or after the required First Security stockholder approval, or Byline or First Security may elect to terminate the merger agreement in certain other circumstances. Please see the section entitled "Description of the Merger Agreement - Termination."

Termination of the merger agreement could negatively affect First Security.

If the merger agreement is terminated there may be various adverse consequences to First Security. For example, First Security's business may have been adversely affected by the failure to pursue other beneficial opportunities due to the focus of management on the merger, without realizing any of the anticipated benefits of completing the merger. In addition, if the merger agreement is terminated and the First Security board seeks another merger or business combination, First Security stockholders cannot be certain that First Security will be able to find a party willing to offer equivalent or more attractive consideration than the consideration Byline has agreed to provide in the merger, or that such other merger or business combination will be completed. Additionally, if the merger agreement is terminated, under certain circumstances First Security may be required to pay Byline a termination fee of $1,653,000. Please see the section entitled "Description of the Merger Agreement - Termination Fee."

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First Security directors and executive officers have interests in the merger different from or in addition to the interests of First Security stockholders generally.

The interests of some of the directors and executive officers of First Security may be different from those of First Security stockholders generally. While the First Security board knew about and considered these interests when making its decision to approve the merger agreement and in recommending that First Security stockholders vote in favor of adopting the merger agreement, First Security stockholders should consider these interests when determining whether to vote to adopt the merger agreement. Please see the section entitled "The Merger - Interests of First Security Directors and Executive Officers in the Merger."

The merger agreement contains provisions that may discourage other companies from trying to acquire First Security for greater merger consideration.

The merger agreement contains provisions that may discourage a third party from submitting a business combination proposal to First Security that might result in greater value to First Security stockholders than the merger with Byline or may result in a potential competing acquirer proposing to pay a lower per share price to acquire First Security than it might otherwise have proposed to pay absent such provisions. These provisions include a general prohibition on First Security from soliciting, or entering into discussions with any third party regarding, any acquisition proposal or offers for competing transactions, subject to certain exceptions relating to the exercise of fiduciary duties by the First Security board. In addition, First Security may be required to pay Byline a termination fee of $1,653,000 upon termination of the merger agreement in certain circumstances involving acquisition proposals for competing transactions. Please see the sections entitled "Description of the Merger Agreement - Termination" and "Description of the Merger Agreement - Termination Fee."

First Security stockholders have appraisal rights in the merger.

If the merger proposal is approved by First Security stockholders, First Security stockholders who do not vote in favor of the approval of the merger proposal and who properly demand payment of fair cash value of their shares of First Security common stock will be entitled to appraisal rights in connection with the merger under Section 262 of the DGCL. However, Byline's obligation to consummate the merger is subject to the condition that the holders of no more than five percent in the aggregate of the outstanding shares of First Security common stock elect to exercise their appraisal rights. Neither First Security nor Byline can predict the number of First Security stockholders who will seek payment of fair cash value of their shares. As a result, if the holders of more than five percent in the aggregate of the outstanding shares of First Security common stock elect to exercise their appraisal rights, the merger may not be completed.

The opinion of First Security's financial advisor delivered to the First Security board prior to the signing of the merger agreement does not reflect changes in circumstances between the date of the opinion and the completion of the merger.

On September 30, 2024, the First Security board received an opinion from D.A. Davidson, its financial advisor, as to the fairness, as of such date, from a financial point of view, to the holders of First Security's outstanding common stock of the exchange ratio to be received by such holders in the merger pursuant to the merger agreement, based upon and subject to the qualifications, assumptions and other matters considered in connection with the preparation of its opinion. D.A. Davidson's written opinion was delivered to the First Security board on September 30, 2024. Changes in the operations and prospects of First Security or Byline may significantly alter the value of First Security or the price of Byline common stock by the time the merger is completed. The opinion does not speak as of the date of this proxy statement/prospectus or the time the merger will be completed or as of any date other than the date of such opinion.

For a description of the opinion that First Security received from its financial advisor, please refer to the section entitled "The Merger - Opinion of First Security's Financial Advisor." A copy of D.A. Davidson's opinion is also attached to this proxy statement/prospectus as Appendix C.

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Byline and First Security will incur transaction and integration costs in connection with the merger.

Each of Byline and First Security has incurred and expects that it will incur nonrecurring costs in connection with consummating the merger. In addition, Byline will incur integration costs following the completion of the merger, including facilities and systems consolidation costs and employment-related costs. There can be no assurances that the expected benefits and efficiencies related to the integration of the businesses will be realized to offset these transaction and integration costs over time. See the risk factor entitled "- Byline may fail to realize the anticipated benefits of the merger." Byline and First Security may also incur additional costs to maintain employee morale and to retain key employees. Byline and First Security will also incur significant legal, financial advisor, accounting, banking and consulting fees, fees relating to regulatory filings and notices, SEC filing fees, printing and mailing fees and other costs associated with the merger. If the merger is not completed, Byline and First Security would have to recognize these expenses without realizing the expected benefits of the merger.

The shares of Byline common stock to be received by First Security stockholders in the merger will have different rights from the shares of First Security common stock.

Upon completion of the merger, First Security stockholders will receive merger consideration consisting of Byline common stock and, accordingly, will become Byline stockholders. As such, their rights as Byline stockholders will be governed by the DGCL and Byline's certificate of incorporation and bylaws. The rights associated with First Security common stock are different from the rights associated with Byline common stock. Please see the section entitled "Comparison of Stockholders' Rights" for a discussion of the different rights associated with Byline common stock.

There is no assurance that Byline will continue paying dividends at the current rate.

Byline's board of directors has adopted a current dividend practice for the payment of a quarterly cash dividend. This practice can be changed at any time at the discretion of the Byline board of directors, and Byline's common stockholders have no contractual or other legal right to dividends. In addition, the other risk factors described in this section could materially reduce the cash available from operations of Byline following the merger, and these outcomes could cause capital not to be available when needed in an amount sufficient to support Byline's current dividend practice. The amount of dividends that Byline may distribute is also subject to restrictions under applicable state law and applicable bank regulatory provisions. If Byline's board of directors were to adopt a change to Byline's current dividend practice that resulted in a reduction in the amount of dividends, such change could have a material and adverse effect on the market price of Byline's common stock.

First Security and Byline 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 First Security and Byline and, consequently, the surviving corporation. These uncertainties may impair First Security's and Byline's ability to attract, retain, and motivate key personnel until the merger is completed, and could cause customers and others that deal with First Security or Byline to change existing business relationships with First Security or Byline, respectively. Retention of certain employees by First Security may be challenging while the merger is pending, as certain employees may experience uncertainty about their future roles with the combined company. If key employees depart because of issues relating to the uncertainty and difficulty of integration, or a desire not to remain with First Security and, ultimately, Byline, the combined company's business could be harmed. In addition, subject to certain exceptions, First Security has agreed to operate its business in the ordinary course and use commercially reasonable efforts to preserve its business organization, employees and business relationships prior to closing. First Security has also agreed not to take certain extraordinary actions while the merger is pending.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

This proxy statement/prospectus contains certain forward-looking information about Byline, First Security, and the combined corporation after the close of the merger, the anticipated benefits and related expenses to be incurred in connection with the merger and the integration of the companies' businesses, as well as certain information about the businesses and strategies of Byline and First Security that is intended to be covered by the safe harbor for "forward-looking statements" provided by the Private Securities Litigation Reform Act of 1995. All statements other than statements of historical fact are forward-looking statements and forward looking statements can be identified by use of the words such as "anticipates," "believes," "expects," "can," "could," "may," "predicts," "potential," "opportunity," "should," "will," "estimate," "plans," "projects," "continuing," "ongoing," "expects," "seeks," "intends" and similar words or phrases, as well as the negatives of such words and phrases. Such statements involve inherent risks, uncertainties and contingencies, many of which are difficult to predict and are generally beyond the control of Byline, First Security and the combined corporation. Readers are cautioned that a number of important factors could cause actual results to differ materially from those expressed in, or implied or projected by, such forward-looking statements. In addition to factors previously disclosed in reports filed by Byline with the SEC, risks and uncertainties for each institution and the combined institution include, but are not limited to:

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All forward-looking statements included in this proxy statement/prospectus are based on information available at the time of the proxy statement/prospectus. Projected or estimated numbers are used for illustrative purposes only and are not forecasts, and actual results may differ materially. A number of important factors could cause actual results to differ materially from those indicated in these forward-looking statements, including those factors identified in "Risk Factors" in "Item 1A. Risk Factors" of Byline's 2023 Annual Report on Form 10-K, which is incorporated by reference herein, as such factors may be updated from time to time in Byline's filings with the SEC.

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Byline and First Security are under no obligation to (and expressly disclaim any such obligation to) update or alter these forward-looking statements, whether as a result of new information, future events or otherwise except as required by law.

FIRST SECURITY SPECIAL MEETING OF STOCKHOLDERS

Date, Time and Place

The special meeting will be held at [ ] [a.m./p.m.], Central Time, on [ ], [ ], virtually only at [ ]. On or about [ ], First Security commenced mailing of this proxy statement/prospectus and the accompanying proxy card to its stockholders entitled to vote at the special meeting.

Purpose of the Special Meeting

At the special meeting, First Security stockholders will be asked to consider and vote upon the following proposals:

First Security stockholder approval is required to complete the merger. First Security does not intend to transact any business at the special meeting other than as listed above.

Recommendation of the First Security Board of Directors

After careful consideration, the First Security board has unanimously approved the merger agreement, the merger and the other transactions contemplated thereby, and unanimously determined that the merger agreement, the merger and the other transactions contemplated thereby are fair to and in the best interests of First Security and its stockholders.

The First Security board recommends that you vote "FOR" the merger proposal and "FOR" the adjournment proposal. Please see the section entitled "The Merger - Recommendation of the First Security Board of Directors and First Security's Reasons for the Merger."

Record Date and Quorum

The First Security board has fixed the close of business on [ ] as the record date for determining the holders of shares of First Security common stock entitled to receive notice of and to vote at the special meeting.

As of the close of business on the record date, there were [674,012] shares of First Security common stock outstanding and entitled to vote at the special meeting held by approximately [31] stockholders of record. Each share of First Security common stock entitles the holder to one vote on each proposal to be considered at the special meeting.

A majority of shares of First Security capital stock issued and outstanding, and entitled to vote, represented in person or by proxy, constitutes a quorum for transacting business at the special meeting.

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Abstentions will be counted as represented at the special meeting for purposes of determining the presence or absence of a quorum for all matters voted on at the special meeting.

As of the close of business on the record date, directors and executive officers of First Security owned and were entitled to vote [303,811] shares of First Security common stock, representing approximately [45.07]% of the shares of First Security common stock outstanding on that date. As of the close of business on the record date, Byline beneficially held [no] shares of First Security common stock.

Arthur Wirtz III, a director of First Security, and Daniel R. Wirtz, First Security's Chairman and President, entered into a voting agreement with Byline solely in their capacities as record and beneficial holders of First Security common stock. Under this agreement, Mr. Arthur Wirtz and Mr. Daniel Wirtz have agreed, subject to certain exceptions, to vote their shares of First Security common stock: (a) in favor of the merger agreement and the transactions contemplated by the merger agreement, including, without limitation, the merger; (b) against any action or agreement which would result in a breach of any term of, or any other obligation of First Security under, the merger agreement or which is reasonably likely to result in any conditions to Byline's obligations under the merger agreement not being fulfilled; and (c) against any action or agreement which would impede or materially delay or interfere with the transactions contemplated by the merger agreement. The [303,811] shares of First Security common stock subject to the voting agreement represented approximately [45.07]% of First Security's outstanding shares of common stock as of [ ], the record date for the special meeting. The voting obligations under the voting agreement will automatically terminate upon the earlier of (a) the effective time and (b) the date of termination of the merger agreement in accordance with its terms.

Required Vote

Treatment of Abstentions; Failure to Vote

For purposes of the special meeting, an abstention occurs when a First Security stockholder attends the special meeting, either in person or by proxy, but abstains from voting.

Abstentions will be counted as represented at the special meeting for purposes of determining the presence or absence of a quorum for all matters voted on at the special meeting.

Voting on Proxies; Incomplete Proxies

Giving a proxy means that a stockholder authorizes the persons named in the proxy to vote such holder's shares at the special meeting in the manner such holder directs. A First Security stockholder may vote by proxy or in person at the special meeting.

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If your shares of First Security common stock are registered directly in your name, you are considered the stockholder of record with respect to these shares. If you hold your shares in your name as a stockholder of record, you may submit your proxy before the special meeting by mail. You must complete, sign, date and return the proxy card in the provided postage-paid envelope. The envelope requires no additional postage if mailed in the United States.

You may also cast your vote live at the virtual special meeting. Please see "- Attending the Special Meeting and Voting in Person" below for further information.

First Security requests that First Security stockholders vote by completing, dating and signing the accompanying proxy and returning it to First Security as soon as possible in the provided postage-paid envelope. When the accompanying proxy is returned properly executed, the shares of First Security common stock represented by it will be voted at the special meeting in accordance with the instructions contained on the proxy card.

If you hold your shares of First Security common stock in your name as a stockholder of record as of the record date, and you sign and return your proxy card without indicating how to vote on any particular proposal, the shares of First Security common stock represented by the proxy will be voted "FOR" the merger proposal and "FOR" the adjournment proposal.

Every stockholder's vote is important. Accordingly, each First Security stockholder should promptly submit a proxy, whether or not the stockholder plans to attend the special meeting.

If you are a stockholder of record and your shares are registered in more than one name, you will receive more than one proxy card. Please complete, sign, date and return each proxy card that you receive.

Revocability of Proxies and Changes to a Stockholder's Vote

You may change your vote at any time before your proxy is voted at the special meeting. You may do this in one of three ways:

If you are a First Security stockholder of record as of the record date and you choose to send a written notice of revocation or to mail a new proxy card, you must submit your notice of revocation or your new proxy to:

First Security Bancorp, Inc.

Attn: Wade Wacholz
7315 W. Grand Avenue
Elmwood Park, IL 60707
(312) 475-6419

Attending the Special Meeting and Voting Live

The special meeting will be held at [ ] [a.m./p.m.], Central Time, on [ ], [ ], virtually only at [ ]. All First Security stockholders as of the close of business on the record date, or their duly appointed proxies, may attend the special meeting.

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Any representative of a stockholder who wishes to attend the special meeting must present acceptable documentation evidencing their authority, acceptable evidence of ownership by the holder of shares of First Security common stock and an acceptable form of identification.

Appraisal Rights

In connection with the merger, First Security stockholders will have the opportunity to exercise appraisal rights in accordance with the procedures set forth in Section 262 of the DGCL. A copy of Section 262 of the DGCL is attached to this proxy statement/prospectus as Appendix B. A dissenting stockholder who does not vote in favor of the merger proposal and who gives notice in writing to First Security at or prior to the special meeting that such holder dissents from the merger and who follows the other required procedures may receive cash in an amount equal to the fair value of their shares of First Security common stock, as determined in accordance with the applicable provisions of the DGCL, in lieu of the merger consideration provided for under the merger agreement. For additional details and information on how to exercise your appraisal rights, please refer to the section entitled "The Merger - Appraisal Rights" and Appendix B to this proxy statement/prospectus. Failure to follow all of the steps required under Section 262 of the DGCL will result in the loss of your appraisal rights.

Solicitation of Proxies

First Security is soliciting proxies for the special meeting from First Security stockholders on behalf of its board of directors. First Security will bear all of the costs of the proxy solicitation for the special meeting, including the costs of preparing, printing and mailing this proxy statement/prospectus to its stockholders. In addition to solicitations by mail, First Security's directors, officers and employees may solicit proxies in person or by telephone, email or other electronic methods without additional compensation.

No Other Matters

First Security stockholder approval is required to complete the merger. First Security does not intend to transact any business at the special meeting other than as listed above.

Questions and Additional Information

If you have any questions or need assistance in voting your shares, please call Wade Wacholz at (312) 475-6419.

INFORMATION ABOUT THE COMPANIES

Byline Bancorp, Inc.

Byline is a bank holding company incorporated in the State of Delaware and headquartered in Chicago, Illinois. Byline's wholly-owned banking subsidiary, Byline Bank, an Illinois state-chartered bank, is a full service commercial bank, and has been a part of the Chicago banking community for over 100 years. Byline Bank offers a broad range of banking products and services to small and medium sized businesses, commercial real estate and financial sponsors and to consumers who generally live or work near its branches. Byline also offers online account opening to consumer and business customers through its website and provide trust and wealth management services to its customers. In addition to its traditional commercial banking business, Byline provides small ticket equipment leasing solutions through Byline Financial Group, a wholly-owned subsidiary of Byline Bank, headquartered in Bannockburn, Illinois, with sales offices in Illinois, and sales representatives in Illinois, Michigan, New Jersey, and New York. Byline also participate in U.S. government guaranteed lending programs and originate U.S. government guaranteed loans. Byline Bank is a leading originator of SBA loans and was the second most active 7(a) and 504 lender in Illinois for the quarter ended June 30, 2024.

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As of September 30, 2024, Byline had consolidated total assets of $9.4 billion, total gross loans and leases outstanding of $6.9 billion, total deposits of $7.5 billion, and total stockholders' equity of $1.1 billion.

Principal Offices and Additional Information

Byline's principal executive office is located at 180 North LaSalle Street, Suite 300, Chicago, Illinois 60601. Byline's telephone number is (773) 244-7000, and Byline's website address is www.bylinebancorp.com. Information on Byline's website is not a part of this proxy statement/prospectus and is not incorporated herein. Byline common stock is traded on the NYSE under the ticker symbol "BY".

Additional information about Byline and its subsidiaries may be found in the documents incorporated by reference into this proxy statement/prospectus. Please also see the section entitled "Where You Can Find More Information."

First Security Bancorp, Inc.

First Security is a one bank holding company headquartered in Elmwood Park, Illinois founded in 1993. First Security is incorporated under the laws of the State of Delaware. First Security is engaged in the banking business through its wholly-owned banking subsidiary, First Security Trust and Savings Bank, an Illinois state-chartered bank that offers commercial and retail banking services at its sole office in Elmwood Park, Illinois. First Security Bank is principally engaged in the business of attracting deposits from the general public and using such deposits, together with borrowings and other funds, to originate commercial, consumer and mortgage loans.

As of September 30, 2024, First Security Bank had total assets of approximately $361.2 million, total loans of approximately $199.7 million and total deposits of approximately $325.5 million.

Principal Office and Additional Information

First Security's principal executive office is located at 7315 W. Grand Avenue, Elmwood Park, IL 60707, and its telephone number at that location is (708) 453-3131.

First Security common stock is privately held and is not quoted on any stock exchange or market. First Security has not declared or paid any cash dividends with respect to its common stock in the last three years.

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THE MERGER

The following is a discussion of the merger and the material terms of the merger agreement between Byline and First Security. You are urged to read carefully the merger agreement in its entirety, a copy of which is attached as Appendix A to this proxy statement/prospectus and incorporated by reference herein. This summary does not purport to be complete and may not contain all of the information about the merger agreement that is important to you. This section is not intended to provide you with any factual information about Byline or First Security. Such information can be found elsewhere in this proxy statement/prospectus and in the public filings Byline and First Security make with the SEC, as described in the section entitled "Where You Can Find More Information."

Terms of the Merger

Transaction Structure

Byline's and First Security's boards of directors have approved the merger agreement and the transactions contemplated thereby. The merger agreement provides that First Security will merge with and into Byline, with Byline as the surviving corporation. Immediately following the merger, First Security Bank, will merge with and into Byline Bank, with Byline Bank as the resulting bank as a wholly-owned subsidiary of Byline.

Merger Consideration

In the merger, each share of First Security common stock outstanding immediately prior to the effective time, other than shares, if any, owned by First Security or Byline (other than shares held in a trust, fiduciary, or nominee capacity or as a result of debts previously contracted), or as to which statutory appraisal rights have been properly exercised and perfected, will be converted into the right to receive the merger consideration consisting of shares of Byline's common stock equal to the quotient (rounded to the nearest ten-thousandth) obtained by dividing (A) 1,468,942 by (B) the number of shares of First Security common stock issued and outstanding immediately prior to the effective time. Based on the number of outstanding shares of First Security common stock as of the date of this proxy statement/prospectus, we expect that the exchange ratio will be equal to 2.1794. In the event that First Security realizes certain recoveries on identified loans sold by it or First Security Bank prior to closing, provided that First Security's closing tangible common equity (as calculated pursuant to merger agreement and set forth on First Security's final closing balance sheet) is equal to or greater than the minimum required as set forth in the merger agreement, then the aggregate merger consideration will be increased as set forth in the merger agreement.

For each fractional share of Byline common stock that would otherwise be issued, Byline will pay cash in an amount equal to the fraction of a share (rounded to the nearest cent) of Byline common stock which the holder would otherwise be entitled to receive multiplied by the Byline closing price. No interest will be paid or accrue on the cash payable to holders in lieu of fractional shares.

The value of the merger consideration will fluctuate with the price of Byline common stock, and the value of the shares of Byline common stock that holders of shares of First Security common stock will receive upon consummation of the merger may be different than the value of the shares of Byline common stock that holders of shares of First Security common stock would receive if calculated on the date Byline and First Security announced the merger, on the date that this proxy statement/prospectus is being mailed to First Security stockholders, or on the date of the special meeting of First Security stockholders.

Based on the closing price of Byline common stock as reported on the NYSE of $26.16 as of September 27, 2024, the trading day prior to the day on which the public announcement of the merger was made, and assuming there are no adjustments to the exchange ratio pursuant to the merger agreement, the implied value of the merger consideration was approximately $57.01 per share of First Security common stock and the implied aggregate transaction value on a fully diluted basis was approximately $38.4 million. Based on the closing price of Byline common stock as reported on the NYSE of $[ ] as of [ ], 2024, the last practicable date before the date

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of this proxy statement/prospectus, and assuming there are no adjustments to the exchange ratio pursuant to the merger agreement, the implied value of the merger consideration was approximately $[ ] per share of First Security common stock and the implied aggregate transaction value on a fully diluted basis was approximately $[ ] million. We urge you to obtain current market quotations for shares of Byline common stock.

Background of the Merger

The First Security board and First Security's senior management regularly review and discuss First Security's business strategy, performance, prospects and operations in the context of the economic environment in Chicago and Illinois more generally, developments in the regulation of financial institutions and the competitive landscape. As part of its strategic planning, the First Security board assesses and evaluates various alternatives for maximizing the value of First Security to its stockholders. Alternatives that are assessed and evaluated include remaining independent or acquiring another financial institution, merging with an institution approximately the same size as First Security or merging with a larger institution. As part of this assessment, the First Security board also evaluates potential headwinds to First Security's continued success as a stand-alone entity, such as economic risk, the potential for increased regulatory burden imposed on banks, factors limiting earnings expansion and the intense competition and ongoing consolidation in the industry.

Beginning in September 2023, representatives of First Security discussed First Security's current market position, potential strategic alternatives and the processes associated therewith with representatives from D.A. Davidson. On October 27, 2023, First Security executed an engagement letter with D.A. Davidson to conduct an evaluation of potential strategic transactions and transaction partners available to First Security. Over the next several weeks, representatives of First Security worked with D.A. Davidson to consider and prepare a list of First Security's desired traits for a potential strategic partner, as well as to assess potential deal terms and structures to maximize value to First Security stockholders. In terms of structure, the First Security board was particularly focused on First Security's stockholders receiving a sufficient amount of stock consideration for the transaction to be structured as a tax-free reorganization.

Representatives of D.A. Davidson met with representatives of First Security in the fall of 2023 to present an illustrative outline of a transaction process, a financial analysis of potential strategic transaction types and an overview of selection metrics for assessing potential partner institutions. Representatives of First Security engaged in significant discussions with representatives of D.A. Davidson throughout the fall of 2023 with respect to First Security's conclusions regarding ideal strategic acquiror traits and transaction structure.

In consultation with D.A. Davidson, the First Security board and First Security's senior management discussed and agreed upon the following desired characteristics of a potential strategic partner:

Based on the foregoing list of desired characteristics of a potential strategic partner, representatives of D.A. Davidson initially identified a number of financial institutions that could be contacted to ascertain their level of interest in acquiring First Security. The First Security board discussed and considered the advantages and disadvantages of the identified financial institutions. After considering various factors, including potential business interruptions, confidentiality risks, staffing issues, negative ramifications for customers and acquiror liquidity and

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market conditions, the First Security board authorized D.A. Davidson to prepare a confidential information memorandum regarding First Security and to contact, on a confidential basis, eleven of the financial institutions identified.

In November 2023, D.A. Davidson reached out to a number of financial institutions regarding their interest in evaluating a possible strategic transaction with First Security. First Security entered into confidentiality agreements with six financial institutions. Non-disclosure agreements were executed with each of these institutions in December 2023. Byline was one of the organizations that entered into a confidentiality agreement to evaluate a possible strategic transaction at that time. On December 20, 2023, representatives from D.A. Davidson and First Security met with principals from Byline to discuss First Security's business model and strategic goals with respect to pursuing a potential merger transaction.

In late January 2024, D.A. Davidson made available certain preliminary information regarding First Security and First Security Bank to all of the institutions that executed a confidentiality agreement to evaluate a possible transaction. This preliminary information included a Confidential Information Memorandum and access to a virtual data room which contained certain preliminary diligence files for prospective transaction counterparties to review.

During the ensuing weeks, the institutions that had executed confidentiality agreements conducted preliminary due diligence on First Security and First Security Bank to determine whether to submit a non-binding indication of interest and pursue further negotiations regarding a potential acquisition of First Security. Of the six institutions that executed confidentiality agreements, two institutions, including Byline, expressed interest in presenting a non-binding indication of interest to proceed with a potential strategic transaction. On February 21, 2024, after reviewing such preliminary information, Byline senior executives met with representatives of D.A. Davidson and certain principals of First Security, including First Security's Chairman Daniel R. Wirtz. This meeting served as an introductory meeting for the parties, and enabled the parties to discuss the drivers and motivations behind a potential transaction as well as to discuss, at a high level, potential terms and conditions of a transaction.

On March 3, 2024, Byline formally requested Stephens Inc. to serve as its exclusive financial advisor to evaluate a possible transaction with First Security. In addition, Byline engaged Vedder Price P.C. to serve as its legal advisor on the transaction. After reviewing the Confidential Information Memorandum and certain other preliminary information regarding First Security and First Security Bank provided as part of the due diligence process, on March 14, 2024, Byline submitted a preliminary indication of interest regarding a possible transaction between First Security and Byline. Following additional discussions with the second potential acquiring institution, that party confirmed that it would not be able to provide an indication of interest within the timeline requested by First Security.

On March 25, 2024, D.A. Davidson met with representatives of the First Security board to evaluate the preliminary indication of interest provided by Byline. D.A. Davidson prepared a summary and analysis of the indication of interest and Byline's common stock, including price, trading volume, analyst estimates and current dividend yield. The First Security board reviewed this analysis and carefully evaluated the relative advantages and any disadvantages of the proposal. The First Security board discussed the advantages of pursuing a transaction with Byline as a partner, including the consideration being offered, the structure of the proposed transaction, the strong liquidity in Byline's public trading market, Byline's geographic footprint and dedication to the Chicagoland area and the belief that Byline's culture would be a good fit for First Security customers and employees.

After a detailed discussion, the First Security board determined that Byline was an attractive strategic partner and the indication of interest presented by Byline provided high potential long-term value to First Security's stockholders, while also providing for ongoing continuity of a high level of service to First Security's customers. The First Security board determined that it was in the best interest of First Security and its stockholders to continue engaging in negotiations with Byline based on the indication of interest.

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On April 18, 2024, First Security provided a revised indication of interest containing certain proposed revisions to Byline's preliminary indication of interest. Between April 18, 2024 and April 30, 2024, representatives of D.A. Davidson and First Security negotiated the terms of the indication of interest with Byline. On April 29, 2024, representatives of D.A. Davidson met with the First Security board to review and further discuss the revised indication of interest and negotiated terms agreed to with Byline. After discussions between the parties and their legal advisors regarding the terms of the indication of interest, on April 30, 2024 the parties entered into a non-binding indication of interest, which included a binding 60 day exclusivity provision.

This final indication of interest provided for, among other things, First Security common stockholders to receive 2.3989 shares of Byline common stock for each share of First Security common stock owned by First Security stockholders. In addition, the indication of interest provided that Byline would redeem First Security's issued and outstanding shares of Series A-1 Nonvoting Preferred Stock, and that the shares of First Security Bank owned by certain minority shareholders would be cashed out or exchanged for shares of First Security common stock immediately prior to closing of the transaction, such that First Security Bank would be a wholly owned subsidiary of First Security prior to closing.

After executing the final indication of interest, Byline was granted access to additional due diligence materials regarding First Security and First Security Bank. Over the next month, Byline and First Security's management and transaction teams engaged in continued discussions regarding the possible strategic transaction. The parties also instructed their financial and legal advisors to engage in discussions to further evaluate a transaction and to begin the process of drafting a definitive merger agreement that would incorporate the terms of the final indication of interest as well as other, customary transaction terms.

During June and July of 2024, the parties continued with a comprehensive due diligence process. On June 24, 2024, Byline and First Security agreed to extend the exclusivity period contemplated by the indication of interest through July 31, 2024.

In late July 2024, based on Byline's comprehensive due diligence findings, particularly related to four loan relationships on First Security's balance sheet, Byline discussed a downward adjustment to the exchange ratio with First Security and its financial advisor. In addition, Byline also introduced a loan recovery concept as a method for First Security to recover all, or a portion, of the lost consideration through the resolution of these identified credits, if resolved prior to the closing of the potential transaction.

Specifically, on July 24, 2024, Byline proposed a revised exchange ratio of 2.1504 shares of Byline common stock for each share of First Security common stock. In addition, Byline proposed that, in the event that First Security realizes certain recoveries on one or more of the subject loan relationships prior to closing of the proposed transaction, the exchange ratio would be adjusted upward. Byline's proposal provided that in no event would recoveries on these loan relationships result in the exchange ratio exceeding the original 2.3989 shares of Byline common stock proposed in the indication of interest for each share of First Security common stock.

On July 26, 2024, Byline's legal counsel, Vedder Price P.C., shared an initial draft of the merger agreement with First Security's legal counsel, Barack Ferrazzano Kirschbaum & Nagelberg LLP.

Through August and September of 2024, the parties continued to negotiate the material terms of the proposed transaction, including the aforementioned revisions to the exchange ratio. The parties also exchanged drafts of the merger agreement and certain related ancillary documents, including the form of voting agreement, and First Security prepared and distributed a set of disclosure schedules listing certain supplemental information and exceptions to its representations and warranties contained in the merger agreement. The parties negotiated, among other items, the representations and warranties that would be made by First Security and Byline, First Security's loan recovery amount(s), certain closing requirements, including without limitation, the requirement that First Security's closing equity equal or exceed a minimum equity level, termination rights, the mechanism to resolve First Security Bank's minority shareholder interest, and other standard terms and conditions. The parties continued conducting comprehensive due diligence on one another through this period. Representatives of First Security and

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its advisors reviewed recent publicly available information concerning Byline, and engaged in discussions regarding the financial and regulatory position of Byline as a strategic transaction partner.

In September 2024, pursuant to a confidentiality agreement entered into by First Security with the minority shareholders of First Security Bank, certain principals of First Security engaged with the minority shareholders to negotiate a transaction which would result in First Security owning 100% of the issued and outstanding common stock of First Security Bank.

On or about September 24, 2024, First Security reached an agreement in principle with the minority shareholders of First Security Bank, pursuant to which the minority shareholders of First Security Bank agreed to exchange their shares of First Security Bank common stock, on a one-for-one basis, for shares of First Security common stock.

On September 26, 2024, after further due diligence, Byline removed one impaired credit from the original list of specifically-identified impaired credits against which recovery could generate an upward adjustment to the exchange ratio. In response, Byline revised its proposed exchange ratio of 2.1504 shares of Byline common stock for each share of First Security common stock upward to 2.1794 shares of Byline common stock for each share of First Security common stock, subject to potential upward adjustment based on recoveries with respect to the three remaining identified impaired credits. On that same day, First Security agreed to the revised exchange ratio of 2.1794 shares of Byline common stock for each share of First Security common stock, including the methodology for potential upward adjustment of the exchange ratio.

Over the next several days, the parties and their respective legal and financial advisors finalized the terms of the definitive merger agreement, related disclosure schedules, bank merger agreement and voting agreement.

On September 30, 2024, the Byline board held a meeting to consider the terms of the merger agreement and the proposed acquisition of First Security. At the board meeting, members of the Byline board and members of Byline's senior management discussed the strategic and business rationale of consummating an acquisition of First Security, and the related merger of First Security Bank with and into Byline Bank, including the factors set forth in more detail in the section titled "- Byline's Reasons for the Merger." Members of Byline's senior management also reported to the Byline board on the results of their due diligence review of First Security and First Security Bank. Also at this meeting, the Byline board reviewed the financial terms of the proposed transaction and other material terms and conditions of the transaction.

Following these discussions with Byline's senior management, and after further deliberations among members of the Byline board, the Byline board determined that the merger agreement and the transactions contemplated thereby, including, without limitation, the merger of First Security with and into Byline and the merger of First Security Bank with and into Byline Bank, were advisable and in the best interests of Byline and its stockholders, and the directors voted unanimously to approve and adopt the merger agreement and the transactions contemplated thereby, including the merger and the bank merger.

In addition, the board of directors of Byline Bank also met on September 30, 2024 to review and approve the bank merger agreement between Byline Bank and First Security Bank pursuant to which First Security Bank will be merged with and into Byline Bank, with Byline Bank as the resulting bank. The Byline Bank board of directors unanimously approved and adopted the bank merger agreement and the bank merger.

Also on September 30, 2024, the First Security board held a special meeting to consider the proposed merger with Byline, including the exchange ratio to be received by First Security stockholders. Each of First Security's directors, as well as representatives of D.A. Davidson and Barack Ferrazzano Kirschbaum & Nagelberg LLP, were present at the meeting. At the meeting, representatives of Barack Ferrazzano Kirschbaum & Nagelberg LLP discussed in detail with the First Security board the fiduciary and legal obligations applicable to directors when considering a sale or merger of First Security. They reviewed the merger agreement and proposed merger transaction with the First Security board in more detail, as well as the terms of the voting agreement.

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During the meeting, D.A. Davidson reviewed the financial aspects of the proposed merger. Based on the closing price of Byline common stock as reported on the NYSE of $26.16 as of September 27, 2024, the trading day prior to the special meeting of the First Security board, and assuming no adjustments to the exchange ratio, the implied value of the merger consideration was approximately $57.01 per share of First Security common stock and the implied aggregate transaction value on a fully diluted basis was approximately $38.4 million. The First Security board also carefully considered the other transaction terms which had been negotiated over the past several months. At the conclusion of their presentation, the representatives of D.A. Davidson rendered to the First Security board an oral opinion to the effect that, as of such date and subject to the procedures followed, assumptions made, matters considered and qualifications and limitations on the review undertaken by D.A. Davidson as set forth in its opinion, the exchange ratio in the proposed merger was fair, from a financial point of view, to the holders of First Security common stock.

After discussion and after considering the proposed terms of the transaction documents, and after taking into account the strategic rationale, financial terms, consideration to be received by First Security stockholders, integration risk and business rationale of consummating a merger with Byline, including the factors described in the section titled "- Recommendation of the First Security Board of Directors and First Security's Reasons for the Merger," the First Security board, having determined that the terms of the merger agreement and the transactions contemplated thereby, including the merger, were fair to and in the best interests of First Security and its stockholders, unanimously approved and declared advisable the merger agreement and the transactions contemplated thereby. The First Security board authorized senior management to finalize and execute the documents, including the merger agreement and the bank merger agreement. The First Security board further directed that the merger agreement be submitted to holders of First Security's common stock for approval, and recommended that holders of First Security common stock vote in favor of the adoption of the merger agreement and the approval of the transactions contemplated thereby.

The First Security Bank board also held a special meeting on September 30, 2024 to review and approve the bank merger agreement. The First Security Bank board of directors unanimously approved and adopted the bank merger agreement and the bank merger at this special meeting.

On September 30, 2024, after market close, the merger agreement and related documents were signed by the parties and Byline announced the transaction via a press release.

Recommendation of the First Security Board of Directors and First Security's Reasons for the Merger

After careful consideration, the First Security board, at a meeting held on September 30, 2024, unanimously determined that the merger agreement, the merger and the other transactions contemplated by the merger agreement were advisable and in the best interests of First Security and its stockholders and approved the merger agreement, the merger and the other transactions contemplated by the merger agreement. Accordingly, the First Security board recommends that First Security stockholders vote "FOR" the merger proposal at the First Security special meeting.

In reaching its decision, the First Security board, with advice from its financial and legal advisors, considered a number of factors, including the following:

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In addition, the First Security board considered a number of risks with respect to the merger, including the following:

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The reasons set forth above are not intended to be exhaustive, but a summary of the material factors considered by the First Security board in approving the merger agreement. Although each member of the First Security board individually considered these and other factors, the First Security board did not collectively assign any specific or relative weights to the factors considered and did not make any determination with respect to any individual factor. The First Security board collectively made its determination with respect to the merger based on the conclusion reached by its members, in light of the factors that each of them considered appropriate, that the merger is in the best interests of First Security and its stockholders. The First Security board realized that there can be no assurance about future results, including results expected or considered in the factors listed above. The First Security board concluded, however, based on its analysis and on discussions with senior management and its financial and legal advisors, that the potential positive factors outweighed the potential risks of entering into the merger agreement.

The foregoing discussion of the information and factors considered by the First Security board is forward- looking in nature and this information should be read in light of the factors described under the section titled "Cautionary Statement Regarding Forward-Looking Statements" in this proxy statement/prospectus.

Opinion of First Security's Financial Advisor

First Security retained D.A. Davidson to act as financial advisor to the First Security board in connection with First Security's consideration of a possible business combination. First Security selected D.A. Davidson to act as its financial advisor because D.A. Davidson is a nationally recognized investment banking firm. In the ordinary course of its investment banking business, D.A. Davidson is regularly engaged in the valuation of financial institutions and their securities in connection with mergers and acquisitions and other corporate transactions.

On October 27, 2023, First Security entered into an engagement agreement with D.A. Davidson on an exclusive basis to render financial advisory and investment banking services to First Security in connection with First Security's review of its financial and strategic alternatives, through sale or merger with another entity. As part of its engagement, D.A. Davidson agreed to assist First Security in analyzing, structuring, negotiating and, if appropriate, effecting a transaction between First Security and another person, corporation, or business entity. D.A. Davidson also agreed to provide the First Security board with an opinion as to the fairness, from a financial point of view, to the holders of the First Security common stock of the exchange ratio to be paid to such holders in the merger. First Security engaged D.A. Davidson because D.A. Davidson is a nationally recognized investment banking firm with substantial experience in transaction similar to the merger and is familiar with First Security and its business. As part of its investment banking business, D.A. Davidson is continually engaged in the valuation of financial institutions and their securities in connection with mergers and acquisitions and other corporate transactions.

On September 30, 2024, the First Security board held a meeting to evaluate the merger. At this meeting, D.A. Davidson reviewed the financial aspects of the merger and rendered an opinion to the First Security board that, as of such date and based upon and subject to assumptions made, procedures followed, matters considered and limitations on the review undertaken, the exchange ratio was fair, from a financial point of view, to the holders of First Security common stock.

The full text of D.A. Davidson's written opinion, dated September 30, 2024, is attached as Appendix C to this proxy statement/prospectus and is incorporated into this proxy statement/prospectus by reference. This description of D.A. Davidson's opinion is qualified in its entirety by reference to the full text of such opinion. First Security's stockholders are urged to read the opinion in its entirety.

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D.A. Davidson's opinion speaks only as of the date of the opinion and D.A. Davidson undertakes no obligation to revise or update its opinion. The opinion is directed to the First Security board and addresses only the fairness, from a financial point of view, to the holders of First Security common stock of the exchange ratio to be paid to such holders in the merger. The opinion does not address, and D.A. Davidson expresses no view or opinion with respect to, (i) the underlying business decision of First Security to engage in the merger, (ii) the relative merits or effect of the merger as compared to any alternative business transaction or strategies that may be or may have been available to or contemplated by First Security or the First Security board, or (iii) any legal, regulatory, accounting, tax or similar matters relating to First Security, its stockholders or relating to or arising out of the merger. The opinion expresses no view or opinion as to any terms or other aspects of the merger, except for the exchange ratio. First Security and Byline determined the exchange ratio through a negotiated process. The opinion does not express any view as to the amount or nature of the compensation to any of First Security's or Byline's officers, directors or employees, or any class of such persons, relative to the exchange ratio, or with respect to the fairness of any such compensation. The opinion has been reviewed and approved by D.A. Davidson's Fairness Opinion Committee in conformity with its policies and procedures established under the requirements of Rule 5150 of the Financial Industry Regulatory Authority.

D.A. Davidson has reviewed the registration statement on Form S-4 of which this proxy statement/prospectus is a part and consented to the inclusion of its opinion to the First Security board as Appendix C to this proxy statement/prospectus and to the references to D.A. Davidson and its opinion contained herein. A copy of the consent of D.A. Davidson is attached as Exhibit 99.1 to the registration statement on Form S-4.

In connection with rendering its opinion, D.A. Davidson reviewed, among other things, the following:

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In arriving at its opinion, D.A. Davidson has, with First Security's consent, assumed and relied upon the accuracy and completeness of all information that was publicly available or supplied or otherwise made available to, discussed with or reviewed by or for D.A. Davidson. D.A. Davidson has not independently verified (nor has it assumed responsibility for independently verifying) such information or its accuracy or completeness. D.A. Davidson has relied on the assurances of management of First Security that they are not aware of any facts or circumstances that would make any of such information, forecasts or estimates inaccurate or misleading. D.A. Davidson has not undertaken or been provided with any independent evaluation or appraisal of any of the assets or liabilities (contingent or otherwise) of First Security. In addition, D.A. Davidson has not assumed any obligation to conduct, nor have it conducted, any physical inspection of the properties or facilities of First Security, and have not been provided with any reports of such physical inspections. D.A. Davidson has assumed that there has been no material change in First Security's business, assets, financial condition, results of operations, cash flows or prospects since the date of the most recent financial statements provided to D.A. Davidson.

With respect to the financial projections and estimates (including information relating to the amounts and timing of the merger costs, cost savings, and revenue enhancements) provided to or otherwise reviewed by or for or discussed with D.A. Davidson, D.A. Davidson has been advised by management of First Security, and have assumed with First Security's consent, that such projections and estimates were reasonably prepared on bases reflecting the best currently available estimates and good faith judgments of management of First Security as to the future financial performance of First Security and the other matters covered thereby, and that the financial results reflected in such projections and estimates will be realized in the amounts and at the times projected. D.A. Davidson assumes no responsibility for and express no opinion as to these projections and estimates or the assumptions on which they were based. D.A. Davidson has relied on the assurances of management of First Security that they are not aware of any facts or circumstances that would make any of such information, projections or estimates inaccurate or misleading.

D.A. Davidson does not specialize in the evaluation of loan and lease portfolios, classified loans or other real estate owned or in assessing the adequacy of the allowance for loan losses with respect thereto, and it did not make an independent evaluation or appraisal thereof, or of any other specific assets, the collateral securing assets or the liabilities (contingent or otherwise) of First Security or Byline or any of their respective subsidiaries. D.A. Davidson has not reviewed any individual loan or credit files relating to First Security or Byline. D.A. Davidson has assumed, with First Security's consent, that the respective allowances for loan and lease losses for both First Security and Byline are adequate to cover such losses and will be adequate on a pro forma basis for the combined entity. D.A. Davidson did not make an independent evaluation of the quality of First Security's or Byline's deposit base, nor has it independently evaluated potential deposit concentrations or the deposit composition of First Security or Byline. D.A. Davidson did not make an independent evaluation of the quality of First Security's or Byline's investment securities portfolio, nor has it independently evaluated potential concentrations in the investment securities portfolio of First Security or Byline.

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D.A. Davidson has assumed that all of the representations and warranties contained in the draft merger agreement and all related agreements are true and correct in all respects material to its analysis, and that the merger will be consummated in accordance with the terms of the draft merger agreement, without waiver, modification or amendment of any term, condition or covenant thereof the effect of which would be in any respect material to D.A. Davidson's analysis. D.A. Davidson also has assumed that all material governmental, regulatory or other consents, approvals, and waivers necessary for the consummation of the merger will be obtained without any material adverse effect on First Security or the contemplated benefits of the merger. Further, D.A. Davidson assumed that the executed merger agreement would not differ in any material respect from the draft merger agreement, dated September 28, 2024, reviewed by D.A. Davidson.

D.A. Davidson has assumed in all respects material to its analysis that First Security and Byline will remain as a going concern for all periods relevant to D.A. Davidson's analysis. D.A. Davidson expresses no opinion regarding the liquidation value of First Security and Byline or any other entity.

D.A. Davidson's opinion is limited to the fairness, from a financial point of view, of the exchange ratio to be paid to the holders of First Security common stock in the merger. D.A. Davidson does not express any view on, and D.A. Davidson's opinion does not address, any other term or aspect of the merger agreement or merger (including, without limitation, the form or structure of the merger) or any term or aspect of any other agreement or instrument contemplated by the merger agreement or entered into in connection with the merger, or as to the underlying business decision by First Security to engage in the merger. Furthermore, D.A. Davidson expresses no opinion with respect to the amount or nature of any compensation to any officers, directors or employees of First Security or Byline, or any class of such persons, relative to the exchange ratio to be paid to the holders of First Security common stock in the merger, or with respect to the fairness of any such compensation. D.A. Davidson's opinion does not take into account individual circumstances of specific holders with respect to control, voting or other rights which may distinguish such holders.

D.A. Davidson expresses no view as to, and its opinion does not address, the relative merits of the merger as compared to any alternative business transactions or strategies, or whether such alternative transactions or strategies could be achieved or are available. In addition, D.A. Davidson's opinion does not address any legal, regulatory, tax or accounting matters, as to which D.A. Davidson understands that First Security obtained such advice as it deemed necessary from qualified professionals.

D.A. Davidson expresses no opinion as to the actual value of Byline common stock when issued in the merger or the prices at which Byline common stock will trade following announcement of the merger or at any future time.

D.A. Davidson has not evaluated the solvency or fair value of First Security or Byline under any state, federal or other laws relating to bankruptcy, insolvency or similar matters. This opinion is not a solvency opinion and does not in any way address the solvency or financial condition of First Security or Byline. D.A. Davidson is not expressing any opinion as to the impact of the merger on the solvency or viability of First Security or Byline or the ability of First Security or Byline to pay their respective obligations when they come due.

Set forth below is a summary of the material financial analyses performed by D.A. Davidson in connection with rendering its opinion. The summary of the analyses D.A. Davidson set forth below is not a complete description of the analysis underlying its opinion, and the order in which these analyses are described below is not indicative of any relative weight or importance given to those analyses by D.A. Davidson. The following summaries of financial analyses include information presented in tabular format. You should read these tables together with the full text of the summary financial analyses, as the tables alone are not a complete description of the analyses.

Unless otherwise indicated, the following quantitative information, to the extent it is based on market data, is based on pricing and market data as of September 26, 2024, and is not necessarily indicative of market conditions after such date. Unless otherwise indicated, financial information is as of June 30, 2024. Note all dollars are in thousands, except per share values, unless otherwise noted.

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Summary of Implied Merger Consideration

D.A. Davidson reviewed a summary of the merger consideration including stock consideration for common stockholders, the exchange ratio, shares to be issued to First Security common stockholders, pro forma ownership amounts for First Security and Byline, and estimated preferred stock redemption proceeds.

Implied Valuation Multiples for First Security based on the Exchange Ratio

D.A. Davidson reviewed the financial terms of the merger. As described in the merger agreement, each share of First Security common stock outstanding immediately prior to the effective time, other than shares, if any, owned by First Security or Byline (other than shares held in a trust, fiduciary, or nominee capacity or as a result of debts previously contracted), or as to which statutory appraisal rights have been properly exercised and perfected, will be converted into and constitute the right to receive the following "Per Common Share Consideration:" such number of fully paid and non-assessable shares of Byline common stock equal to the quotient (rounded to the nearest ten-thousandth) obtained by dividing (A) 1,468,942 by (B) the number of shares of Company Common Stock issued and outstanding immediately prior to the effective time; provided, however, that in the event First Security has less than 674,012 shares of common stock issued and outstanding at closing, then the exchange ratio will be equal to 2.1794. The exchange ratio is subject to adjustments pursuant to Section 3.1(a)(2) of the merger agreement. The terms and conditions of the merger are more fully described in the merger agreement. Based upon financial information as of or for the 12 month period ended June 30, 2024, the closing price of Byline common stock on September 26, 2024, and other financial and market information described below, D.A. Davidson calculated the following transaction ratios:

Sensitivity Analysis on Byline Stock Price - Merger Consideration

D.A. Davidson analyzed the sensitivity of First Security's Per Common Share Consideration based on movement in Byline's stock price of +/- 20.00% versus Byline's closing stock price of $26.17 as of September 26, 2024. Assuming Byline's stock price of $26.17, First Security's Per Common Share Consideration was $57.03, or $38.4 million on an aggregate basis. Assuming Byline's stock price increased by 20.00%, First Security's Per Common Share Consideration increased to $68.44, or $46.1 million on an aggregate basis. Conversely, assuming Byline's stock price decreased by 20.00%, First Security's Per Common Share Consideration decreased to $45.63, or $30.8 million on an aggregate basis. Additionally, D.A. Davidson analyzed the sensitivity of First Security's Per Common Share Consideration based on Byline's 52-week high stock price, 52-week low stock price, and various volume weighted average prices.

Implied First Security Per Common Share Consideration

D.A. Davidson analyzed the implied First Security Per Common Share Consideration based on Byline's reported trading stock prices and the exchange ratio over the last 12 months.

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Historical Market Performance of Byline

D.A. Davidson reviewed the historical trading prices of Byline's common stock and certain stock indices, including the S&P 500, S&P U.S. BMI Banks and the NASDAQ Bank indices over the last 12 months, five years, and since Byline's IPO. D.A. Davidson compared the stock price performance of Byline with the performance of the S&P 500, S&P U.S. BMI Banks and the NASDAQ Bank indices as follows:

Trading Volume Analysis of Byline

D.A. Davidson reviewed the average daily trading volume and average daily dollar volume of Byline over the last week, month, three months and one-year periods ($ amounts in thousands):

Stock Price and Volume History of Byline

D.A. Davidson reviewed Byline closing stock prices, one day stock price changes and trading volume for the prior 60 trading days.

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Byline Dividends

D.A. Davidson reviewed Byline's dividend yield over the last 12 months, dividend payout ratio and dividends per share over the last five quarters and over the next three quarters using publicly available analyst estimates. In addition, D.A. Davidson analyzed the amount of pro forma dividends per share First Security's common stockholders will receive per current First Security common share owned on a quarterly and annual basis using the exchange ratio.

Byline Comparable Companies Analysis

D.A. Davidson used publicly available information to compare selected financial and market trading information for Byline and a group of 12 financial institutions selected by D.A. Davidson which: (i) were headquartered in the Midwestern United States including: Iowa, Illinois, Indiana, Kansas, Kentucky, Michigan, Minnesota, Missouri, North Dakota, Nebraska, Ohio, South Dakota and Wisconsin; (ii) had their common stock listed on the NASDAQ and NYSE; (iii) had assets between $7.5 billion and $12.5 billion; and (iv) were not pending merger targets. These 12 financial institutions were as follows:

The analysis compared the financial condition and market performance of Byline and the 12 financial institutions identified above based on publicly available financial information for Byline and the 12 financial institutions as of and for the 12 months ended June 30, 2024. The market trading information for Byline and the 12 financial institutions identified above is as of September 26, 2024. The analysis also compared the 2024 and 2025 earnings per share multiples for Byline and the 12 financial institutions identified above based on publicly available analyst earnings estimates for Byline and its peers. The analysis did not reflect the impact from pending acquisitions or acquisitions closed after June 30, 2024. The table below shows the results of this analysis.

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