UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q/A

(Amendment No. 1)

For the quarterly period ended June 30, 2012.

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)

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  x    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 (§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).    YES  x    NO  ¨

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 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 Rule 12b-2 of the Exchange Act)     YES  ¨    NO  x

At August 6, 2012 the Company had outstanding 115,986,494 shares of common stock, par value $0.01 per share.

Explanatory Note

Stillwater Mining Company (the “Company”) is filing this Amendment No. 1 to its Form 10-Q originally filed with the Securities and Exchange Commission on August 8, 2012 (the “Original Filing”). This Amendment is being filed solely for the purpose of correcting typographical errors in the certifications filed as Exhibit 32.1 and 32.2 to the Original Filing.

ITEM 6. Exhibits

(a) The following exhibits are filed as part of this report.

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 thereunto duly authorized.