UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

Amendment No. 2

For the fiscal year ended December 31, 2011

OR

For the transition period from              to             

Commission file number: 000-51222

DEXCOM, INC.

(Exact name of Registrant as Specified in its Charter)

Registrant’s Telephone Number, including area code: (858) 200-0200

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

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

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

Indicate by check mark if the Registrant is not required to file reports pursuant to Section 13 or 15(d) of the Exchange 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 website, 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 Rule 405 of Regulation S-K is not contained herein, and will not be contained, to the best of Registrant’s knowledge, in definite 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 Rule 12b-2 of the Exchange Act).     Yes  ¨    No  x

As of June 30, 2011, the aggregate market value of the registrant’s common stock held by non-affiliates of the registrant was approximately $960,461,796 based on the closing sales price as reported on the NASDAQ Global Market.

Indicate the number of shares outstanding of each of the issuer’s classes of common stock, as of the latest practicable date.

DexCom, Inc.

Table of Contents

EXPLANATORY NOTE

DexCom, Inc. (the “DexCom”) filed its Annual Report on Form 10-K for the year ended December 31, 2011 with the Securities and Exchange Commission (“SEC”) on February 23, 2012 (the “Annual Report”), and filed an Amendment No. 1 to the Annual Report on Form 10-K/A with the SEC on March 28, 2012 (the “Amendment No. 1”). DexCom is filing this Amendment No. 2 as an exhibit-only filing to the Annual Report (the “Amendment No. 2”), to re-file Exhibit 10.26 in response to comments received from the staff of the SEC regarding a request for confidential treatment of certain portions of Exhibit 10.26 originally filed with the Annual Report. This Amendment No. 2 is being filed solely to re-file Exhibit 10.26 and to amend and restate the Item 15 (Exhibits and Financial Statement Schedules) and the Exhibit Index included in the Annual Report. In connection with the filing of this Amendment No.2, DexCom is also including certain currently dated certifications of our Chief Executive Officer and Chief Financial Officer as required by Rule 12b-15 under the Securities Exchange Act of 1934, as amended. This Amendment No. 2 makes reference to the date of the Annual Report, and DexCom has not updated or amended the disclosures contained herein to reflect events that have occurred since the filing of the Annual Report, or modified or updated the disclosures contained in the Annual Report and Amendment No. 1 in any way other than as specifically set forth in this Amendment No. 2. Accordingly, this Amendment No. 2 should be read in conjunction with the Annual Report, Amendment No. 1 and other filings made by DexCom with the SEC subsequent to our filing of the Annual Report.

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PART IV

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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.

POWER OF ATTORNEY

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 and in the capacities and dates indicated.

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EXHIBIT INDEX 

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