COLUMBIA BANKING SYSTEM INC

Rule 10b5-1 or Non-Rule 10b5-1 Trading Arrangements

On March 7, 2025, Torran Nixon, Senior Executive Vice President of the Company, terminated a trading arrangement for the sale of shares of Columbia common stock (a “Rule 10b5-1 Trading Plan”) that was intended to satisfy the affirmative defense conditions of Exchange Act Rule 10b5-1(c). Mr. Nixon's Rule 10b5-1 Trading Plan was adopted on December 3, 2024 and provided for the sale of up to 7,461 shares of Columbia common stock pursuant to the terms of the plan. As of the date of the plan termination, Mr. Nixon had sold 4,481 shares of Columbia common stock under the plan.

During the three months ended March 31, 2025, other than disclosed above, none of our directors or officers adopted orterminated 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 intended 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).