COLUMBIA BANKING SYSTEM, INC.

Insider Trading Arrangements and Policies

A significant portion of the compensation of our executive officers is delivered in the form of equity awards, including time-vested restricted stock unit awards, and time-vested stock awards that vest ratably over three-year periods, and performance-based restricted stock unit awards that typically vest over a three-year performance period. This compensation design is intended to align executive compensation with the performance experienced by our shareholders. Following the delivery of shares of our common stock under those equity awards, once any applicable service-, time- or performance-based vesting standards have been satisfied, our executive officers from time to time engage in the open-market sale of some of those shares. Certain of our officers have elected to have shares withheld at the time of vesting to cover withholding taxes.

Our directors and executive officers may also engage from time to time in other transactions involving our securities. Transactions in our securities by our directors and executive officers are required to be made in accordance with our Insider Trading and Information Policy, which, among other things, requires that the transactions be in accordance with applicable U.S. federal securities laws that prohibit trading while in possession of material non-public information. Rule 10b5-1 under the Exchange Act provides an affirmative defense that enables pre-arranged transactions in securities in a manner that avoids concerns about initiating transactions at a future date while possibly in possession of material non-public information. Our Insider Trading and Information Policy permits our directors and executive officers to enter into trading plans designed to comply with Rule 10b5-1.

On November 25, 2025, Torran Nixon, Senior Executive Vice President of the Company, adopted a trading arrangement for the sale of shares of stock (a “Rule 10b5-1 Trading Plan”) that is intended to satisfy the affirmative defense conditions of Exchange Act Rule 10b5-1(c). Mr. Nixon's Rule 10b5-1 Trading Plan provides for termination on November 2, 2026 (subject to earlier termination for certain customary termination events) and for the sale of up to 7,461 shares of common stock.

During the year ended December 31, 2025, other than disclosed above, none of our directors or officers adopted or terminated a trading plan intended to satisfy Rule 10b5-1 or any "non-Rule 10b5-1 trading arrangement," as defined in Item 408 of Regulation S-K.

Certain of our officers and directors have made elections to participate in, and are participating in, our 401(k) and profit sharing plan and have made, and may from time to time make, elections to (i) have shares withheld to cover withholding taxes or (ii) have dividends from Columbia common stock reinvested into Columbia common stock, (iii) have a portion of their 401(k) account contributions used to purchase Columbia common stock, or (iv) participate in the employee stock purchase plan, which may be designed to satisfy the affirmative defense conditions of Rule 10b5-1 under the Exchange Act or may constitute non-Rule 10b5-1 trading arrangements (as defined in Item 408(c) of Regulation S-K).