UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

(Amendment No. 1)

(Mark One)

for the fiscal year ended December 31, 2012

or

for the transition period from             to             .

Commission file number 0-21810

GENTHERM INCORPORATED

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (248) 504-0500

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 whether the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  x

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  x

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 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 the 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, 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  x

The aggregate market value of the registrant’s Common Stock held by non-affiliates of the registrant, computed by reference to the average bid and asked prices of such Common Stock as of the last business day of the registrant’s most recently completed second fiscal quarter, June 30, 2012, was $330,154,000. For purposes of this computation, the registrant has excluded the market value of all shares of its Common Stock reported as being beneficially owned by executive officers and directors and holders of more than 10% of the Common Stock on a fully diluted basis of the registrant; such exclusion shall not, however, be deemed to constitute an admission that any such person is an “affiliate” of the registrant.

As of March 15, 2013, there were 33,206,494 issued and outstanding shares of Common Stock of the registrant.

EXPLANATORY NOTE

This Amendment No. 1 on Form 10-K/A (this “Amendment”) amends our Annual Report on Form 10-K for the fiscal year ended December 31, 2012, that was filed with the Securities and Exchange Commission on March 15, 2013 (the “Original Filing”). The sole purpose of this Amendment is to correct the dates of the signatures of directors signing the Original Filing. The Original Filing was signed by the directors of the Company on March 15, 2013; however, the typeset signature page to the Original Filing incorrectly showed the dates of such signatures as March 15, 2012 due to a typographical error in production. In order to correct the error, the required conforming signatures are included with this Amendment.

Pursuant to Rule 12b-15 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”), this Form 10-K/A also contains new certifications pursuant to Section 302 of the Sarbanes-Oxley Act of 2002. Because no financial statements have been included in this Form 10-K/A and this Form 10-K/A does not contain or amend any disclosure with respect to Items 307 and 308 of Regulation S-K, paragraphs 3, 4 and 5 of the certifications have been omitted.

Except as described above, no other changes are made to the Original Filing. The Original Filing continues to speak as of the date of the Original Filing or the dates specified therein. Unless expressly stated, this Amendment does not reflect events occurring after the filing of the Original Filing, nor does it modify or update in any way the disclosures contained in the Original Filing.

PART IV

The following documents are filed as part of this report:

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.

Pursuant to the requirements of the Securities Exchange Act of 1934, this report has been signed below by the following persons on behalf of the registrant in the capacities and on the dates indicated.

Each person whose signature appears below hereby authorizes Daniel R. Coker and Barry G. Steele, or any of them, as attorneys-in-fact to sign on his behalf, individually, and in each capacity stated below, and to file all amendments and/or supplements to this Annual Report on Form 10-K.

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.