UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-Q/A

(Amendment No. 1)

For the Quarterly Period Ended March 31, 2015

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:

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.

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 April 27, 2015, there were 29,801,681 shares of the registrant’s Common Stock outstanding.

Explanatory Note

This Amendment No. 1 to Form 10-Q/A (this “Amendment”) amends the Tandem Diabetes Care, Inc. (the “Company”) Quarterly Report on Form 10-Q for the period ended March 31, 2015, previously filed with the Securities and Exchange Commission on April 30, 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 II

Item 6. Exhibits

The following documents are filed as exhibits to this Amendment:

2

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.

3

Exhibit Index

4