Exhibit 23.3

CONSENT OF INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

We consent to the incorporation by reference in this Amendment No. 2 to the Registration Statement No. 333-216039 on Form S-4 of our reports dated February 28, 2017, relating to the consolidated financial statements of Columbia Banking System, Inc. and its subsidiaries (the “Company”), and the effectiveness of the Company’s internal control over financial reporting, appearing in the Annual Report on Form 10-K of Columbia Banking System, Inc. for the year ended December 31, 2016, and to the reference to us under the heading “Experts” in the Joint Proxy Statement/Prospectus, which is part of this Registration Statement.

/s/ Deloitte &Touche LLP

Seattle, Washington

April 20, 2017