BANNER CORP

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, DC 20549

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

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☐ 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

BANNER CORPORATION

(Name of registrant as specified in its charter)

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Proxy Statement

Dear Shareholder:

You are cordially invited to attend the annual meeting of shareholders of Banner Corporation, which will be held on Wednesday, May 20, 2026, at 10:00 a.m., Pacific Time. Our 2026 annual meeting of shareholders will be a virtual meeting conducted solely online via live webcast. There is no physical location for the annual meeting. The online meeting format will facilitate remote shareholder attendance and participation, including the ability to vote your shares electronically and submit questions during the meeting.

You can attend the meeting by visiting https://meetnow.global/M4KQTQY. To participate in the annual meeting, registered shareholders will need the control number included on their proxy card and all other shareholders will need to follow the instructions that accompanied their proxy materials.

The Notice of Annual Meeting of Shareholders and Proxy Statement describe the formal business to be transacted at the meeting. Directors and officers of Banner Corporation, as well as a representative of Baker Tilly US, LLP, our independent registered public accounting firm, will be available to respond to shareholder questions. We intend to answer questions pertinent to our business immediately following the formal meeting of shareholders.

It is important that your shares are represented at the meeting, whether or not you attend the meeting virtually and regardless of the number of shares you own. To make sure your shares are represented, we urge you to vote promptly. You may vote your shares via the Internet or a toll-free telephone number, or by completing and mailing your proxy card. If you attend the virtual meeting using your shareholder control number, you may vote your shares at that time even if you have previously submitted your proxy.

We hope you can attend the virtual annual meeting.

Sincerely,

Mark J. Grescovich

President and Chief Executive Officer

Notice of Annual Meeting of Shareholders

Voting Matters

At or before the 2026 Annual Meeting of Shareholders, we ask that you vote the following items:

Your Vote is Important — How to Vote

To ensure your shares are represented at the meeting, please vote via the Internet or telephone, or sign, date and mail your proxy card. Your proxy is solicited on behalf of the Board of Directors. The proxy will not be used if you attend and vote at the virtual annual meeting. Regardless of the number of shares you own, your vote is very important. Please act today. Our 2026 annual meeting of shareholders will be a virtual meeting conducted solely online via live webcast. The online meeting format facilitates remote shareholder attendance and participation, including the ability to vote shares electronically and submit questions during the meeting. To participate in the annual meeting, registered shareholders will need the control number included on their proxy card and all other shareholders will need to follow the instructions that accompanied their proxy materials. A list of Banner shareholders as of the record date will be available for inspection, via the “Shareholder” login on the virtual meeting website noted above, by any shareholder (or their agents or attorneys) beginning ten days prior to the meeting and continuing through the meeting. Beneficial owners of shares held in street name who wish to participate in the meeting as a “Shareholder” must register with Computershare no later than 5:00 p.m., Eastern Time, on May 14, 2026. (Late submissions may be processed to the extent feasible, but registration cannot be guaranteed in time for participation in the meeting.) To request registration, forward the proxy-granting email from your broker or email an image of your legal proxy to legalproxy@computershare.com, with a subject line of “Legal Proxy,” or mail your request to Computershare, Banner Corporation Legal Proxy, P.O. Box 43001; Providence, RI 02940-3001. Computershare will confirm your registration and provide you with a unique control number which enables full participation in the meeting.

BY ORDER OF THE BOARD OF DIRECTORS

SHERREY LUETJEN

Secretary, Banner Corporation

Walla Walla, Washington