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) March 21, 2013

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:

¨ Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

¨ Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

¨ Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

¨ Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Section 7 – Regulation FD

Floyd Chadee, Senior Vice President and Chief Financial Officer of StanCorp Financial Group, Inc. (“StanCorp”) will present at the J.P. Morgan 2013 Insurance Conference on March 21, 2013. Mr. Chadee will focus on StanCorp’s strategy and financial results, as well as current market conditions. A copy of the presentation materials is attached hereto as Exhibit 99.1 and furnished herewith.

Financial measures that exclude after-tax net capital gains and losses and accumulated other comprehensive income (loss) (“AOCI”) are non-GAAP (Generally Accepted Accounting Principles in the United States) measures. To provide investors with a broader understanding of earnings, the Company provides net income per diluted share excluding after-tax net capital gains and losses, along with the GAAP measure of net income per diluted share, because capital gains and losses are not likely to occur in a stable pattern.

Return on average equity excluding after-tax net capital gains and losses from net income and AOCI from equity is furnished along with the GAAP measure of net income return on average equity because management believes providing both measures gives investors a broader understanding of return on equity. Measuring return on average equity without AOCI excludes the effect of market value fluctuations of the Company’s fixed maturity securities associated with changes in interest rates and other market data. Management believes that measuring return on average equity without AOCI is important to investors because the turnover of the Company’s portfolio of fixed maturity securities may not be such that unrealized gains and losses reflected in AOCI are ultimately realized. Furthermore, for the purpose of calculating this ratio, management believes exclusion of AOCI provides investors with a better measure of return.

Section 9 – Financial Statements and Exhibits

(d) Exhibits

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