哥伦比亚银行系统美股招股说明书(2025-06-16版)

To the Shareholders of Columbia Banking System, Inc. and the Stockholders of Pacific Premier Bancorp, Inc. MERGER PROPOSED—YOUR VOTE IS VERY IMPORTANT On behalf of the boards of directors of Columbia Banking System, Inc. (“Columbia”) and Pacific Premier Bancorp, Inc. (“Pacific Premier”), we arepleased to enclose the accompanying joint proxy statement/prospectus relating to the proposed acquisition by Columbia of Pacific Premier. We arerequesting that you take certain actions as a holder of Columbia common stock (a “Columbia shareholder”) or as a holder of Pacific Premier commonstock (a “Pacific Premier stockholder”). On April23, 2025, Columbia, Pacific Premier and Balboa Merger Sub, Inc. (“Merger Sub”) entered into an Agreement and Plan of Merger (as may beamended, modified or supplemented from time to time in accordance with its terms, the “merger agreement”), pursuant to which Columbia will acquirePacific Premier. The transaction will create a premier West Coast banking franchise with approximately $69billion in total assets and approximately$57billion in total deposits, operating through locations spanning Oregon, Washington, California, Idaho, Nevada, Arizona, Colorado and Utah. Pursuant to the merger agreement, Merger Sub will merge with and into Pacific Premier (the “merger”), with Pacific Premier as the survivingcorporation. Immediately following the merger, Pacific Premier will merge with and into Columbia (the “second step merger” and together with themerger, the “mergers”), with Columbia as the surviving entity. Promptly following the completion of the mergers, Pacific Premier Bank, NationalAssociation, a national banking association and a wholly owned subsidiary of Pacific Premier, and Umpqua Bank, an Oregon state-charteredcommercial bank and a wholly owned subsidiary of Columbia, will merge, with Umpqua Bank as the surviving bank. In the merger, Pacific Premier stockholders will receive 0.9150 (the “exchange ratio”) of a share of Columbia common stock for each share of PacificPremier common stock they own. Based on the closing price of Columbia common stock on the Nasdaq Global Select Market (“Nasdaq”) on April22,2025, the last trading day before the public announcement of the mergers, the exchange ratio represented approximately $20.83 in value for each shareof Pacific Premier common stock, representing merger consideration of approximately $2.0billion on an aggregate basis. Based on the closing price ofColumbia common stock on Nasdaq on June13, 2025, the last practicable trading day before the date of the accompanying joint proxystatement/prospectus, of $22.60, the exchange ratio represented approximately $20.68 in value for each share of Pacific Premier common stock. Columbia shareholders will continue to own their existing shares of Columbia common stock following the mergers. The value of the Columbia common stock at the time of completion of the merger could be greater than, less than or the same as the value of Columbiacommon stock on the date of the accompanying joint proxy statement/prospectus. We urge you to obtain current market quotations of Columbiacommon stock (Nasdaq trading symbol “COLB”) and Pacific Premier common stock (Nasdaq trading symbol “PPBI”). We expect the mergers, taken together, will qualify as a “reorganization” within the meaning of Section368(a) of the Internal Revenue Code of 1986, asamended (the “Code”). Accordingly, Pacific Premier stockholders generally will not recognize any gain or loss for federal income tax purposes on theexchange of shares of Pacific Premier common stock for Columbia common stock in the merger, except with respect to any cash received by PacificPremier stockholders in lieu of fractional shares of Columbia common stock. Based on the number of shares of Pacific Premier common stock outstanding or reserved for issuance as of June12, 2025, Columbia expects to issueapproximately 89.3million shares of Columbia common stock to Pacific Premier stockholders in the aggregate in the merger. We estimate that existingColumbia shareholders Table of Contents will own approximately 70% and former Pacific Premier stockholders will own approximately 30% of the common stock of Columbia following thecompletion of the mergers. The special meeting of Columbia shareholders will be held on July 21, 2025 at 9:00 a.m., Pacific Time. The special meeting of Pacific Premierstockholders will be held on July 21, 2025 at 9:00 a.m., Pacific Time. At our respective special meetings, in addition to other business, Columbia willask its shareholders to approve the issuance of shares of Columbia common stock to Pacific Premier stockholders pursuant to the merger agreement, andPacific Premier will ask its stockholders to adopt the merger agreement. Information about these meetings and the mergers is contained in thisdocument. We urge you to read this document carefully and in its entirety. Each of our boards of directors unanimously recommends that holders of commo