署名银行美股招股说明书(2025-06-12版)

4,282,210 shares Common stock The selling stockholders identified in this prospectus supplement are offering up to an aggregateof 4,282,210 shares of common stock, par value $0.01 per share (the “Common Stock”), of BylineBancorp, Inc. (the “Company,” “we,” “our” or “us”). We will not receive any proceeds from the saleof the Common Stock in this offering. Subject to the completion of this offering, we will purchase from the underwriter 418,235 sharesof common stock offered by the selling stockholders in this offering (the “Stock Repurchase”) at aprice per share equal to the price per share at which the underwriter will purchase the sharesfrom the selling stockholders in this offering. The underwriter will not receive any compensationfor the shares of Common Stock purchased by us from the underwriter in the Stock Repurchase.The closing of the Stock Repurchase is expected to be concurrent with the closing of this offering.The completion of this offering is not conditioned upon the completion of the Stock Repurchase.See “Prospectus Supplement Summary—Stock Repurchase.” The underwriter has allocated 51,333 shares of Common Stock being offered in this offering tocertain of our directors at the public offering price and on the same terms as the other purchasersin this offering. The Common Stock is traded on the New York Stock Exchange (“NYSE”) under the symbol “BY”.The last reported sales price of our Common Stock on the NYSE on June 10, 2025 was $26.72per share. Shares of Common Stock are not savings accounts or deposits and are not insured by theFederal Deposit Insurance Corporation or any other government agency. Investing in our securities involves risk. See “Risk Factors” beginning on pageS-9of thisprospectus supplement and on page 7 of the accompanying base prospectus and the otherrisk factors incorporated by reference into this prospectus supplement and theaccompanying base prospectus. Neither the Securities and Exchange Commission (“SEC”) nor any state securitiescommission has approved or disapproved of these securities or passed upon theadequacy or accuracy of this prospectus supplement or the accompanying prospectus.Any representation to the contrary is a criminal offense. The underwriter expects to deliver the shares of Common Stock to purchasers against paymenton or about June12, 2025 through the book-entry facilities of The Depository Trust Company. Sole Book-Running Manager J.P. Morgan The date of this prospectus supplement is June11, 2025 Table of contents Prospectus supplement PageAbout this prospectus supplementS-1Forward-looking statementsS-3Prospectus supplement summaryS-5The offeringS-7Risk factorsS-9Use of proceedsS-11Selling stockholderS-12MaterialU.S.federal income tax consequences to non-U.S.holdersS-13UnderwritingS-17Validity of the common stockS-28ExpertsS-28Where you can find more informationS-29Incorporation by referenceS-29 Prospectus About this prospectus1Where you can find more information2Incorporation by reference2Cautionary note regarding forward-looking statements4About Byline Bancorp, Inc.5Risk factors7Use of proceeds8Description of capital stock8Selling stockholders12Plan of distribution13Validity of securities14Experts14 About this prospectus supplement This document is in two parts. The first part is this prospectus supplement, which describes thespecific terms of this offering. The second part, the accompanying base prospectus, gives moregeneral information, some of which may not apply to this offering. Generally, when we use theterm “prospectus,” we are referring to both parts combined. You should read this entireprospectus supplement and the accompanying base prospectus, as well as the documentsincorporated by reference herein and therein that are described under “Where You Can Find MoreInformation” in this prospectus supplement and in the accompanying base prospectus. In theevent that the information varies between this prospectus supplement and the accompanyingbase prospectus, you should rely on the information contained in this prospectus supplement. In making an investment decision, prospective investors must rely on their own examination ofthe Company and the terms of the offering, including the merits and risks involved. Prospectiveinvestors should not construe anything in this prospectus as legal, business or tax advice. Eachprospective investor should consult its own advisors as needed to make its investment decisionand to determine whether it is legally permitted to purchase the securities under applicable lawsand regulations. Neither we, the selling stockholders nor the underwriter have authorized anyone to provideyou with any information or to make any representations other than those contained in thisprospectus supplement and the accompanying base prospectus, or any free writingprospectuses prepared by or on behalf of us or to which we have referred you. Neither we,the selling stockholders nor the underwriter take responsibility for, and ne