UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

FOR THE FISCAL YEAR ENDED DECEMBER 31, 2013.

or

FOR THE TRANSITION PERIOD FROM                    TO                     

Commission File Number 1-13053

STILLWATER MINING COMPANY

(Exact name of registrant as specified in its charter)

1321 Discovery Drive,

Billings,

Montana 59102

(Address of principal executive offices and zip code)

(406) 373-8700

(Registrant’s telephone number, including area code)

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

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

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.  x    YES  ¨    NO

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  x    NO

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.  x    YES  ¨    NO

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Date File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).  x    YES  ¨    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.  x

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, 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. (Check one):

Indicate by check mark whether the registrant is a shell company (as defined in Exchange Act Rule 12b-2).  ¨    YES  x    NO

At June 28, 2013, the aggregate market value of the registrant’s voting and non-voting common equity held by non-affiliates of the registrant was approximately $1.3 billion based on the closing sale price as reported on the New York Stock Exchange. There were 119,629,040 shares of common stock, par value $0.01 per share, outstanding on February 25, 2014.

DOCUMENTS INCORPORATED BY REFERENCE

Certain information required in Part III of this Annual Report on Form 10-K is incorporated herein by reference to the Registrant’s Proxy Statement for its 2014 Annual Meeting of Stockholders.

EXPLANATORY NOTE

The registrant has prepared this Amendment No. 1 (“Amendment”) on Form 10-K/A to its Annual Report on Form 10-K for the fiscal year ended December 31, 2013 (the “Form 10-K”) for the purpose of filing Exhibits 10.65 and 10.66 to the Form 10-K. No revisions are being made to the Company’s financial statements and this Amendment does not reflect events occurring after the filing of the Form 10-K, or modify or update those disclosures that may be affected by subsequent events, and no other changes are being made to any other disclosure contained in the Form 10-K.

This Amendment is an exhibit-only filing to add two exhibits that were inadvertently omitted from the original filing. This Amendment is being filed solely to add Exhibit 10.65 and Exhibit 10.66. Except for the addition of Exhibits 10.65 and 10.66, this Amendment does not update any exhibits as originally filed.

In addition, as required by Rule 12b-15 under the Securities Exchange Act of 1934, new certifications by our principal executive officer and principal financial officer are filed herewith as exhibits to this Amendment.

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the Registrant has duly caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.