UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, D.C. 20549 Large accelerated filer ☒ Accelerated filer ☐ Non-accelerated filer ☐ Smaller reporting company ☐ Emerging growth company ☐ FORM 10-K ☒ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended December 31, 2020 ☐ TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(D) OF THE SECURITIES EXCHANGE ACT OF 1934 Commission File Number 000-20288 COLUMBIA BANKING SYSTEM, INC. (Exact name of regist ...