COLUMBIA BANKING SYSTEM, INC.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

SCHEDULE 14A

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a)

of the Securities Exchange Act of 1934 (Amendment No.)

Check the appropriate box:

Preliminary Proxy Statement

Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

Definitive Proxy Statement

Definitive Additional Materials

Soliciting Material Pursuant to §240.14a-12

COLUMBIA BANKING SYSTEM, INC.

(Name of Registrant as Specified In Its Charter)

(Name of Person(s) Filing Proxy Statement, if Other Than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

No fee required

Fee paid previously with preliminary material

Fee computed on table in exhibit required by Item 25(b) per Exchange Act Rules 14a-6(i)(1) and 0-11

Columbia Banking System, Inc.

1301 A Street

Tacoma, Washington 98402-4200

March 31, 2026

To our Shareholders:

During 2025, we continued to advance our strategic priorities while delivering solid operating performance and consistent,

repeatable financial results. A defining milestone of the year was the announcement and closing of our strategic acquisition of

Pacific Premier Bancorp, Inc. (“Pacific Premier”), followed by a seamless systems conversion in January 2026. The acquisition

completed our Western footprint, bolstering our position as the preeminent regional bank in the Northwest and improving our

competitive position in other key western markets, most notably Southern California, where we now hold a top 10 deposit market

share position. We also contributed to our operational momentum through de novo growth, opening new locations in Arizona,

California, Colorado, and Oregon, funded by resources set aside from our 2024 expense initiatives. Our actions prioritized

optimized performance and long-term shareholder value creation as we support our associates, customers, and communities.

The Board has nominated 12 individuals for election or re-election, as applicable, at this year’s annual meeting of shareholders,

which will be held on Thursday, May 14, 2026, at 10:00 a.m. Pacific Time. Directors Maria M. Pope and S. Mae Fujita Numata have

not been nominated for re-election in connection with their desire to retire from the Columbia Board effective as of the Annual

Meeting. Ms. Pope and Ms. Numata served most recently as, respectively, Lead Independent Director of Columbia and Chair of the

Columbia Bank Trust Committee until its dissolution in December 2025. Ms. Pope and Ms. Numata have provided 12 and 14 years

of dedicated board service to the Company, respectively, and we are deeply grateful for their many years of leadership, insight,

and stewardship. Directors Steven R. Gardner, M. Christian Mitchell and Jaynie Miller Studenmund joined the Columbia Board in

connection with the acquisition of Pacific Premier. Mr. Gardner is the former board chair, chief executive officer and president of

Pacific Premier, and Mr. Mitchell and Ms. Studenmund are the former chairs of Pacific Premier’s audit committee and compensation

committee, respectively. They bring valuable banking and public company expertise to the Columbia Board and have been

nominated for election at the annual meeting.

The annual meeting on May 14, 2026, will be conducted exclusively via live webcast. We believe this format enables engagement

with our shareholders regardless of size, resources or physical location. You will be able to attend the meeting online, vote your

shares electronically and submit questions during the meeting by visitingwww.virtualshareholdermeeting.com/COLB2026.

To participate in the virtual meeting, you will need the 16-digit control number included on your Notice, proxy card or voting

instruction form. The meeting webcast will begin promptly at 10:00 a.m. Pacific Time. We encourage you to access the meeting

prior to the start time. Online check-in will begin at 9:45 a.m. Pacific Time, and you should allow ample time for the check-in

procedures. If you experience any difficulties while accessing the virtual meeting during the check-in or meeting time, a technical

assistance phone number will be made available on the virtual meeting registration page starting at 9:45 a.m. Pacific Time on the

date of the meeting.

At the meeting, in addition to voting on the matters set forth in the proxy statement, you will have the opportunity to hear

management discuss developments in our business and industry during the past year and to ask questions pertinent to the

meeting. You will find additional information concerning our company and its operations, including its audited financial statements,

in the Annual Report for the year ended December 31,2025, which is available on our website at

www.columbiabankingsystem.com in the “Quarterly Results” section.

Whether or not you plan to attend the virtual meeting, please take the time to vote online, by telephone or by completing

and mailing the proxy card (if you received one) as soon as possible. Your opinion and your vote are important to us. Voting

by proxy will not prevent you from voting online if you attend the meeting, but it will ensure that your vote is counted if you are

unable to attend.

Chair, Chief Executive Officer and President

Notice of Annual Meeting of Shareholders

Time and Date

10:00 a.m. Pacific Time on Thursday, May 14, 2026

Virtual Meeting

www.virtualshareholdermeeting.com/COLB2026

Visitwww.virtualshareholdermeeting.com/COLB2026 and enter the control number

found on your Notice, proxy card or instruction form.

How To Participate

You are entitled to vote at the annual meeting and at any adjournments or postponements

of the meeting if you were a shareholder at the close of business on March 20, 2026.

Record Date

Items of Business

Board

Recommendation

Page

Reference

The purposes of the meeting are as follows:

(1)To elect the 12 nominees for director named in the accompanying Proxy Statement

to serve on the Board of Directors until the 2027 annual meeting of shareholders or

until their successors have been elected and have qualified.

FOR

(2)To approve, on an advisory basis, the compensation of the Company’s named

executive officers.

FOR

(3) To ratify the appointment of Deloitte & Touche LLP as the Company’s independent

registered public accounting firm for the fiscal year ending December 31,2026.

FOR

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NOTE: In their discretion, the proxies are authorized to vote upon such other business as may properly come before the Annual Meeting and

any adjournments or postponements thereof.

Voting

By Internet:

By Telephone:

By Mail:

To vote before the meeting, visitwww.proxyvote.com

By Toll Free Number:

1-800-690-6903

Follow the instructions

on your proxy card

To vote at the meeting, visit

www.virtualshareholdermeeting.com/COLB2026

By Order of the Board of Directors,

Dated: March 31, 2026

Kumi Yamamoto Baruffi

EVP, General Counsel and Corporate Secretary / Tacoma, Washington