UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K

CURRENT REPORT

Pursuant to Section 13 or 15(d) of the

Securities Exchange Act of 1934

Date of Report (Date of Earliest Event Reported): December 11, 2015

Banner Corporation

(Exact name of registrant as specified in its charter)

10 S. First Avenue

Walla Walla, Washington 99362

(Address of principal executive offices and zip code)

(Registrant’s telephone number, including area code) (509) 527-3636

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions.

Banner Corporation (the “Company”) is filing information for the purpose of updating and, to the extent the information overlaps, superseding certain sections in its Annual Report on Form 10-K for the fiscal year ended December 31, 2014, filed with the U.S. Securities and Exchange Commission (the “SEC”) on March 16, 2015. This information includes the risk factor and business (including management) disclosure contained in its prior public filings, including those discussed under the headings “Item 1A. Risk Factors” and “Item 1: Business.” The Company included the updated risk factors and certain updated information describing its business, management and indebtedness in the Registration Statement it filed on December 11, 2015 with the SEC, and that information has been also been filed as Exhibits 99.1, 99.2, 99.3 and 99.4 to this Current Report on Form 8-K and is incorporated herein by reference.

Forward-Looking Statements

Some of the statements in the documents incorporated by reference herein constitute forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. These statements relate to our financial condition, liquidity, results of operations, plans, objectives, future performance or business. Forward-looking statements are not statements of historical fact, are based on certain assumptions and are generally identified by use of the words “believes,” “expects,” “anticipates,” “estimates,” “forecasts,” “intends,” “plans,” “targets,” “potentially,” “probably,” “projects,” “outlook” or similar expressions or future or conditional verbs such as “may,” “will,” “should,” “would” and “could.” Forward-looking statements include statements with respect to our beliefs, plans, objectives, goals, expectations, assumptions and statements about future economic performance and projections of financial items. These forward-looking statements are subject to known and unknown risks, uncertainties and other factors that could cause actual results to differ materially from the results anticipated or implied by our forward-looking statements, including, but not limited to: expected revenues, cost savings, synergies and other benefits from the merger of Banner Bank and Siuslaw Bank and of the merger of Banner Bank and AmericanWest and costs or difficulties relating to integration matters, including but not limited to customers, systems and employee retention, might be greater than expected or occur at different times than expected; the credit risks of lending activities, including changes in the level and trend of loan delinquencies and write-offs and changes in our allowance for loan losses and provision for loan losses that may be impacted by deterioration in the housing and commercial real estate markets and may lead to increased losses and non-performing assets, and may result in our allowance for loan losses not being adequate to cover actual losses and require us to materially increase our reserves; changes in economic conditions in general and in Washington, Idaho, Oregon, Utah and California in particular; changes in the levels of general interest rates and the relative differences between short and long-term interest rates, loan and deposit interest rates, our net interest margin and funding sources; fluctuations in the demand for loans, the number of unsold homes, land and other properties and fluctuations in real estate values in our market areas; secondary market conditions for loans and our ability to sell loans in the secondary market; results of examinations of us by the Federal Reserve Board and of our bank subsidiaries by the FDIC, the Washington State Department of the DFI or other regulatory authorities, including the possibility that any such regulatory authority may, among other things, institute an informal or formal enforcement action against us or any of our bank subsidiaries which could require us to increase our reserve for loan losses, write-down assets, change our regulatory capital position or affect our ability to borrow funds, or maintain or increase deposits, or impose additional requirements and restrictions on us, any of which could adversely affect our liquidity and earnings; legislative or regulatory changes that adversely affect our business including changes in regulatory policies and principles, or the interpretation of regulatory capital or other rules, including changes related to Basel III; the impact of the Dodd-Frank Act and the implementing regulations; our ability to attract and retain deposits; increases in premiums for deposit insurance; our ability to control operating costs and expenses; the use of estimates in determining fair value of certain of our assets and liabilities, which estimates may prove to be incorrect and result in significant changes in valuation; difficulties in reducing risk associated with the assets from our combined securities portfolio and our and AmericanWest’s loan portfolios on our balance sheet; staffing fluctuations in response to product demand or the implementation of corporate strategies that affect our work force and potential associated charges; the failure or security breach of computer systems on which we depend; our ability to retain key members of our senior management team; costs and effects of litigation, including settlements and judgments; our ability to implement our business strategies; future goodwill impairment due to changes in our business, changes in market conditions, or other factors; our ability to manage loan delinquency rates; increased competitive pressures among financial services companies; changes in consumer spending, borrowing and savings habits; the availability of resources to address changes in laws, rules, or regulations or to respond to regulatory actions; our ability to pay dividends on our common stock, non-voting common stock and any preferred stock, and interest or principal payments on our junior subordinated debentures; adverse changes in the securities markets; inability of key third-party providers to perform their obligations to us; changes in accounting policies and practices, as may be adopted by the financial institution regulatory agencies or the Financial Accounting Standards Board, including additional guidance and interpretation on accounting issues and details of the implementation of new accounting methods; the economic impact of war or any terrorist activities; other economic, competitive, governmental, regulatory, and technological factors affecting our operations, pricing, products and services; and other risks detailed from time to time in our filings with the SEC, including under the heading “Risk Factors” in Exhibit 99.1 hereto which is incorporated herein by reference.

Any forward-looking statements are based upon management’s beliefs and assumptions at the time they are made. We do not undertake and specifically disclaim any obligation to update any forward-looking statements included or incorporated by reference in this prospectus or the reasons why actual results could differ from those contained in such statements, whether as a result of new information, future events or otherwise. These risks could cause our actual results to differ materially from those expressed in any forward-looking statements by, or on behalf of, us. In light of these risks, uncertainties and assumptions, the forward-looking statements discussed or incorporated by reference in this Form 8-K might not occur, and you should not put undue reliance on any forward-looking statements.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

Exhibit Index