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): August 18, 2015

STANCORP FINANCIAL GROUP, INC.

(Exact name of registrant as specified in its charter)

(971) 321-7000

(Registrant’s telephone number, including area code)

No Change

(Former name, former address and former fiscal year, if changed since last report)

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:

Item 8.01 – Other Events.

Pursuant to the Agreement and Plan of Merger, dated as of July 23, 2015, (the “Merger Agreement”) by and among StanCorp Financial Group, Inc., an Oregon corporation (the “Company”), Meiji Yasuda Life Insurance Company, a mutual insurance company (seimei hoken sōgo-kaisha) organized under the Laws of Japan (“Meiji Yasuda”), and MYL Investments (Delaware) Inc., a Delaware corporation and wholly-owned subsidiary of Meiji Yasuda, during the post-signing solicitation period (which ended at 12:01 a.m., New York City time, on August 18, 2015), Goldman, Sachs & Co., the Company’s financial advisor, held discussions with 20 potential acquirers on behalf of the Company to solicit interest in a possible alternative transaction. During such time, none of these parties executed a confidentiality agreement or otherwise expressed an interest in pursuing a transaction, and no other party proposed a possible alternative transaction. The Company has informed Meiji Yasuda that no person executed a confidentiality agreement and that the board of directors of the Company has not determined that any person is an “Excluded Party” under the Merger Agreement.

Additional Information and Where to Find It

This filing may be deemed solicitation material in respect of the proposed acquisition of the Company by Meiji Yasuda. In connection with the proposed merger transaction, the Company will file with the SEC and furnish to the Company’s shareholders a proxy statement and other relevant documents. BEFORE MAKING ANY VOTING DECISION, THE COMPANY’S SHAREHOLDERS ARE URGED TO READ THE PROXY STATEMENT IN ITS ENTIRETY WHEN IT BECOMES AVAILABLE AND ANY OTHER DOCUMENTS TO BE FILED WITH THE SEC IN CONNECTION WITH THE PROPOSED MERGER OR INCORPORATED BY REFERENCE IN THE PROXY STATEMENT BECAUSE THEY WILL CONTAIN IMPORTANT INFORMATION ABOUT THE PROPOSED MERGER AND THE PARTIES TO THE MERGER. The Company’s shareholders will be able to obtain a free copy of documents filed with the SEC at the SEC’s website at http://www.sec.gov. In addition, the Company’s shareholders may obtain a free copy of the Company’s filings with the SEC from the Company’s investor relations website at www.stancorpfinancial.com or by directing a request to: StanCorp Financial Group, Inc., 1100 SW Sixth Avenue, Portland, OR 97204, Attn: Jeff Hallin, Vice President, Investor Relations and Capital Markets, (971) 321-6127, jeff.hallin@standard.com.

The directors, executive officers and certain other members of management and employees of the Company may be deemed “participants” in the solicitation of proxies from shareholders of the Company in favor of the proposed merger. Information regarding the persons who may, under the rules of the SEC, be considered participants in the solicitation of the shareholders of the Company in connection with the proposed merger will be set forth in the proxy statement and the other relevant documents to be filed with the SEC. You can find information about the Company’s executive officers and directors in its Annual Report on Form 10-K filed with the SEC on February 26, 2015 and in its definitive proxy statement filed with the SEC on Schedule 14A on March 23, 2015.

Forward-looking Statements

Statements in this Current Report on Form 8-K and the exhibits furnished or filed herewith that relate to future results and events are forward-looking statements based on the Company’s current expectations. Actual results and events in future periods may differ materially from those expressed or implied by these forward-looking statements because of a number of risks, uncertainties and other factors. All statements other than statements of historical fact are statements that could be deemed forward-looking statements. Risks, uncertainties and assumptions include, but are not limited to: (1) the occurrence of any event, change or other circumstances that could give rise to the termination of the merger agreement; (2) the inability to complete the proposed merger due to the failure to obtain shareholder approval for the proposed merger or the failure to satisfy other conditions to completion of the proposed merger, including that a governmental entity may prohibit, delay or refuse to grant approval for the consummation of the transaction; (3) risks related to disruption of management’s attention from the Company’s ongoing business operations due to the transaction; and (4) the effect of the announcement of the proposed merger on the Company’s relationships with its distributors, operating results and business generally.

Actual results may differ materially from those indicated by such forward-looking statements. In addition, the forward-looking statements represent the Company’s views as of the date on which such statements were made. The Company anticipates that subsequent events and developments will cause its views to change. However, although the Company may elect to update these forward-looking statements at some point in the future, it specifically disclaims any obligation to do so. These forward-looking statements should not be relied upon as representing the Company’s views as of any date subsequent to the date hereof. Additional factors that may cause results to differ materially from those described in the forward-looking statements are set forth in the Company’s Annual Report on Form 10–K for the fiscal year ended December 31, 2014, which was filed with the SEC on February 26, 2015, under the heading “Item 1A—Risk Factors,” and in subsequent reports on Forms 10–Q and 8–K filed with the SEC by the Company.

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.