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UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

For the fiscal year ended December 31, 2016 or

For the transition period from                     to                     

Commission File Number 000-19289

Registrant’s telephone number, including area code:

(614) 464-5000

Securities registered pursuant to Section 12(b) of the Act: None

Securities registered pursuant to Section 12(g) of the Act:

Common Shares, without par value

Indicate by check mark if the Registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  ý

Indicate by check mark if the Registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  ý

Indicate by check mark whether the Registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the Registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  ý    No  ¨

Indicate by check mark whether the Registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T during the preceding 12 months (or for such shorter period that the Registrant was required to submit and post such files).    Yes  ý    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K is not contained herein, and will not be contained, to the best of Registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.  ¨

Indicate by check mark whether the Registrant is a large accelerated filer, an accelerated filer, or a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer”, “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act.

Indicate by check mark whether the Registrant is a shell company (as defined in Rule 12b-2 of the Act).    Yes  ¨    No  ý

As of June 30, 2016, the last business day of the Registrant’s most recently completed second fiscal quarter, the aggregate market value (based on the closing sales price on that date) of the voting stock held by non-affiliates of the Registrant was $344,055,419.

On February 24, 2017, the Registrant had 41,976,998 Common Shares outstanding.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the Registrant’s Proxy Statement relating to the annual meeting of shareholders to be held May 5, 2017 (the “2017 Proxy Statement”), which will be filed within 120 days of December 31, 2016, are incorporated by reference into Part III of this Form 10-K.

Index to Annual Report on Form 10-K for the year ended December 31, 2016

IMPORTANT INFORMATION REGARDING FORWARD-LOOKING STATEMENTS

All statements, other than statements of historical facts, included in this Annual Report on Form 10-K (this “Form 10-K”) of State Auto Financial Corporation (“State Auto Financial” or “STFC”) or incorporated herein by reference, including, without limitation, statements regarding State Auto Financial’s future financial position, business strategy, budgets, projected costs, goals and plans and objectives of management for future operations, are forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995. Forward-looking statements generally can be identified by the use of forward-looking terminology such as “may,” “will,” “expect,” “intend,” “estimate,” “anticipate,” “project,” “believe” or “continue” or the negative thereof or variations thereon or similar terminology. Forward-looking statements speak only as the date the statements were made. Although State Auto Financial believes that the expectations reflected in forward-looking statements have a reasonable basis, it can give no assurance that these expectations will prove to be correct. Forward-looking statements are subject to risks and uncertainties that could cause actual events or results to differ materially from those expressed in or implied by the statements. For a discussion of the most significant risks and uncertainties that could cause State Auto Financial’s actual results to differ materially from those projected, see “Risk Factors” in Item 1A of this Form 10-K. Except to the limited extent required by applicable law, State Auto Financial undertakes no obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.