July 27, 2017

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 proposed offer and sale by Tandem Diabetes Care, Inc., a Delaware corporation (the “Company”), of up to an aggregate of $15,000,000 of shares of the Company’s common stock, par value $0.001 per share (the “Shares”), pursuant to a Registration Statement on Form S-3 (File No. 333-200686) (the “Registration Statement”), which was originally filed under the Securities Act of 1933, as amended (the “Securities Act”) with the Securities and Exchange Commission (“SEC”) on December 2, 2014 and declared effective by the SEC on December 19, 2014, the base prospectus contained in the Registration Statement (the “Base Prospectus”), and the prospectus supplement relating to the proposed offer and sale of the Shares filed with the SEC on July 27, 2017 pursuant to Rule 424(b) of the rules and regulations under the Securities Act (the “Prospectus Supplement”, and together with the Base Prospectus, the “Prospectus”). We understand that the Shares are proposed to be offered and sold by the Company through Piper Jaffray & Co. (the “Agent”) pursuant to an equity distribution agreement among the Company and the Agent (the “Equity Distribution Agreement”).

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 without independent investigation or verification.

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, the Prospectus and the Equity Distribution Agreement, will be validly issued, fully paid and non-assessable.

We render this opinion only with respect to the General Corporation Law of the State of Delaware, and we express no opinion herein concerning the application or effect of the laws of any other jurisdiction.

We hereby consent to the use of this opinion as Exhibit 5.1 to the Company’s Current Report on Form 8-K filed with the SEC on the date hereof, which is incorporated by reference into the Registration Statement, and further consent to the reference to us in the Registration Statement and any amendments thereto. 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 offer and sale of the Shares pursuant to the Registration Statement and is not to be relied upon for any other purpose or delivered to or relied upon by any other

Securities Registered under Registration Statement on Form S-3

July 27, 2017

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person without our prior written consent. This opinion is rendered as of the date hereof and based solely on our understanding of facts in existence as of such date after the examination described in this opinion.  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.