UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-K/A

(AMENDMENT NO. 1)

For the fiscal year ended December 31, 2014

or

For the transition period from                      to                     

Commission File Number 001-36189

Tandem Diabetes Care, Inc.

(Exact name of registrant as specified in its charter)

(858) 366-6900

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.    Yes  ¨    No  x

Indicate by check 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 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 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 Act).    Yes  ¨    No  x

As of June 30, 2014, the aggregate market value of the registrant’s common stock held by non-affiliates was approximately $180 million based on the closing price for the common stock of $16.26 on that date. Shares of common stock held by each executive officer, director, and their affiliated stockholders have been excluded from this calculation as such persons may be deemed to be affiliates. This determination of affiliate status is not necessarily a conclusive determination for other purposes.

As of February 20, 2015, there were 23,714,990 shares of the registrant’s common stock outstanding.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement for the 2015 Annual Meeting of Stockholders, filed with the Securities and Exchange Commission on April 9, 2015, were incorporated by reference into Part III, Items 10-14 of our Annual Report on Form 10-K for the year ended December 31, 2014, filed with the Securities and Exchange Commission on February 24, 2015. 

Explanatory Note

This Amendment No. 1 to Form 10-K/A (this “Amendment”) amends the Tandem Diabetes Care, Inc. Annual Report on Form 10-K for the year ended December 31, 2014, previously filed with the Securities and Exchange Commission on February 24, 2015 (the “Original Filing”). This Amendment is being filed solely to include revised Exhibits 31.1 and 31.2, which replace the previously filed versions of those exhibits, to include certain statements required by Item 601(b)(31) of Regulation S-K inadvertently omitted when previously filed. Each certification, as corrected by this Amendment, was true and correct as of the date of the Original Filing.

Except as described as above, no changes have been made to the Original Filing. The Original Filing continues to speak as of the date of the Original Filing, and the Company has not updated the disclosures contained therein to reflect any events which occurred as of a date subsequent to the date of the Original Filing.  

PART IV

Item 15. Exhibits, Financial Statement Schedules

The following documents are filed as exhibits to this Amendment:

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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 Annual Report to be signed on its behalf by the undersigned, thereunto duly authorized.

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Exhibit Index

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