Exhibit 5.1

November 1, 2013

Tandem Diabetes Care, Inc.

11045 Roselle Street

San Diego, California 92121

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the filing by Tandem Diabetes Care, Inc., a Delaware corporation (the “Company”), of a Registration Statement on Form S-1 (Filing No. 333-191601) (as amended, and as may be further amended or supplemented, the “Registration Statement”) with the Securities and Exchange Commission, covering an underwritten public offering of up to 7,143,000 shares (the “Shares”) of the Company’s common stock, par value $0.001, which includes up to 1,071,450 Shares that may be sold by the Company pursuant to the exercise of an over-allotment option granted to the underwriters by the Company. The Shares are being sold to the several underwriters named in, and pursuant to, an underwriting agreement among the Company and such underwriters.

In connection with the preparation of this opinion, we have examined such documents and considered such questions of law as we have deemed necessary or appropriate. We have assumed the authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof and the genuineness of all signatures. As to questions of fact material to our opinions, we have relied upon the certificates of certain officers of the Company.

Based on the foregoing, we are of the opinion that the Shares have been duly authorized and, when issued and sold in the manner described in the Registration Statement, will be validly issued, fully paid and non-assessable.

We are members of the Bar of the State of California and, accordingly, do not purport to be experts on or to be qualified to express any opinion herein concerning, nor do we express any opinion herein concerning, any laws other than the laws of the State of California and the General Corporation Law of the State of Delaware.

We hereby consent to the use of this opinion as Exhibit 5.1 to the Registration Statement, and to the use of our name under the caption “Legal Matters” in the Registration Statement. In giving such consent, we do not hereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.

This opinion is intended solely for use in connection with the issuance and sale of the Shares pursuant to the Registration Statement and is not to be relied upon for any other purpose. We assume no obligation to advise you of any fact, circumstance, event or change in the law or the facts that may hereafter be brought to our attention whether or not such occurrence would affect or modify the opinions expressed herein.

Very truly yours,

STRADLING YOCCA CARLSON & RAUTH, P.C.

/s/ Stradling Yocca Carlson & Rauth, P.C.