Exhibit 5.1

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The Trade Desk, Inc.

42 N. Chestnut Street

Ventura, California 93001

| Re: |  | Form S-1 Registration Statement File No. 333-216105

Public Offering of up to 5,750,000 Shares of Class A Common Stock of The Trade Desk, Inc.

Ladies and Gentlemen:

We have acted as special counsel to The Trade Desk, Inc., a Delaware corporation (the “Company”), in connection with the registration statement on Form S-1 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on February 16, 2017 (Registration No. 333-216105) (as amended, the “Registration Statement”). The Registration Statement relates to the registration of up to 5,750,000 shares (including 750,000 shares that may be purchased by the underwriters pursuant to an option to purchase additional shares) of Class A common stock, par value $0.000001 per share (“Class A Common Stock”), which are being offered by certain stockholders (the “Selling Stockholders”) of the Company (collectively, the “Shares”). The term “Shares” shall include any additional shares of common stock registered by the Company pursuant to Rule 462(b) under the Act in connection with the offering contemplated by the Registration Statement. This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or related prospectus (the “Prospectus”), other than as expressly stated herein with respect to the issue of the Shares.

As such counsel, we have examined such matters of fact and questions of law as we have considered appropriate for purposes of this letter. With your consent, we have relied upon certificates and other assurances of officers of the Company and others as to factual matters without having independently verified such factual matters. We are opining herein as to the General Corporation Law of the State of Delaware, and we express no opinion with respect to any other laws.

February 21, 2017

Page 2

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, the Shares have been duly authorized by all necessary corporate action of the Company and, upon the conversion of the shares of Class B common stock of the Company, par value $0.000001 per share, into shares of Class A Common Stock in accordance with the amended and restated certificate of incorporation of the Company, the Shares will be validly issued, fully paid and nonassessable.

This opinion is for your benefit in connection with the Registration Statement and may be relied upon by you and by persons entitled to rely upon it pursuant to the applicable provisions of the Act. We consent to your filing this opinion as an exhibit to the Registration Statement and to the reference to our firm in the Prospectus under the heading “Legal Matters.” We further consent to the incorporation by reference of this letter and consent into any post-effective amendment to the Registration Statement filed pursuant to Rule 462(b) with respect to the Shares. In giving such consent, we do not thereby admit that we are in the category of persons whose consent is required under Section 7 of the Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Latham & Watkins LLP