BY Byline Bancorp Inc

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.