FirstSun Capital Bancorp美国招股说明书（2026年2月6日版）

This joint proxy statement/prospectus Supplement (this “Supplement”) is dated as of February 6, 2026 and is first beingmailed to stockholders of FirstSun Capital Bancorp (“FirstSun”) and First Foundation Inc. (“First Foundation”) on or about February This Supplement amends and supplements the joint proxy statement/prospectus of FirstSun and First Foundation, datedJanuary 15, 2026 (the “joint proxy statement/prospectus”), which was previously mailed to stockholders of FirstSun and FirstFoundation on or about January 16, 2026, in connection with their respective special meetings of stockholders scheduled to be held on FirstSun filed the joint proxy statement/prospectus with the U.S. Securities and Exchange Commission (the “SEC”) as part ofa registration statement on Form S-4. To the extent of any conflict between this Supplement and the joint proxy statement/prospectus, the terms of this Supplementshall control with respect to the matters described herein. Capitalized terms used but not otherwise defined in this Supplement have the This Supplement does not contain complete information about FirstSun, First Foundation or the merger. You should read thisSupplement together with the joint proxy statement/prospectus and the annexes thereto, as well as the documents incorporated by IMPORTANT INFORMATION FOR STOCKHOLDERS Stockholders who have not yet voted or submitted a proxy are urged to do so as promptly as possible. If you have already submitted a proxy, you do not need to take any action unless you wish to revoke or change your proxy,which you may do in the manner described in the joint proxy statement/prospectus. The date, time and location of the special meetings of stockholders of FirstSun and First Foundation have not changed. Neither theU.S.Securities and Exchange Commission nor any state securities commission has approved ordisapproved of the merger, the issuance of shares of FirstSun common stock in connection with the merger, or the othermatters described in the joint proxy statement/prospectus, or passed upon the adequacy or accuracy of the disclosure The shares of FirstSun common stock to be issued in the merger are not savings accounts, deposits or other obligationsof any bank or savings association and are not insured by the Federal Deposit Insurance Corporation or any other PURPOSE OF THIS SUPPLEMENT The purpose of this Supplement is to describe and reflect an amendment to the Agreement and Plan of Merger, dated October27, 2025 (the “merger agreement”), by and between FirstSun and First Foundation, which amendment updates the form of the As described below, the merger agreement has been amended to revise and narrow certain elective conversion provisionsapplicable to the non-voting common stock, without affecting the automatic conversion provisions. The amendment does not change Except as expressly described in this Supplement, the joint proxy statement/prospectus remains unchanged, and the boards ofdirectors of FirstSun and First Foundation continue to unanimously recommend that their respective stockholders vote “FOR” each of BACKGROUND AND SUMMARY OF AMENDMENT On February 6, 2026, FirstSun and First Foundation entered into Amendment No. 1 to the Merger Agreement (the “mergeragreement amendment”). The merger agreement amendment amends Exhibit E to the merger agreement, which sets forth the form of the Certificate ofAmendment to FirstSun’s Amended and Restated Certificate of Incorporation that will create a class of non-voting common stock. As amended: the ability of holders of non-voting common stock to elect to convert all or a portion of its shares of non-voting commonstock into common stock to the extent, and only to the extent, that such conversion would not cause the holder and itsaffiliates, together with any other persons whose ownership must be aggregated with such holder under applicable banking it has been replaced with a provision that permits conversion of non-voting common stock into common stock at the holder’selection (as distinguished from automatic conversion) only if an action by FirstSun reduces the holder’s percentage ownershipof a class of voting securities, and only to the extent that such conversion does not result in the holder acquiring a greater The merger agreement amendment does not modify any other provisions of the merger agreement and does not affect: the merger consideration, the exchange ratio, the voting mechanics, or the proposals being submitted for approval at the special meetings. SUPPLEMENTAL DISCLOSURES TO THE JOINT PROXY STATEMENT/PROSPECTUS The following information supplements and amends the joint proxy statement/prospectus and should be read in conjunctionwith the joint proxy statement/prospectus, which should be read in its entirety. Except as described below, the joint proxy Page 45 -Summary of Key Characteristics of the Non-Voting Common Stock The subsection titled “Summary of Key Characteristics of the Non-Voting C