Exhibit 5.1

650 Town Center Drive, 20th Floor

Costa Mesa, California 92626-1925

Tel: +1.714.540.1235 Fax: +1.714.755.8290

www.lw.com

FIRM / AFFILIATE OFFICES

Barcelona    Moscow

Beijing    Munich

Boston    New York

Brussels    Orange County

Century City    Paris

Chicago    Riyadh

Dubai    Rome

Düsseldorf    San Diego

Frankfurt    San Francisco

Hamburg    Seoul

Hong Kong    Shanghai

Houston    Silicon Valley

London    Singapore

Los Angeles    Tokyo

Madrid    Washington, D.C.

Milan    

June 2, 2017

Obalon Therapeutics, Inc.

5421 Avenida Encinas, Suite F

Carlsbad, CA 92008

Re:	Registration Statement on Form S-8: 951,155 shares of Common Stock, par value $0.001 per share, of Obalon Therapeutics, Inc.

Ladies and Gentlemen:

We have acted as special counsel to Obalon Therapeutics, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing by the Company with the Securities and Exchange Commission (the “Commission”) of a registration statement on Form S–8 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the issuance of up to 951,155 shares of the Company’s common stock, par value $0.001 per share (the “Shares”), which may be issued pursuant to the Company’s 2016 Equity Incentive Plan and 2016 Employee Stock Purchase Plan (together, the “Plans”). This opinion is being furnished in connection with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act, and no opinion is expressed herein as to any matter pertaining to the contents of the Registration Statement or the prospectus forming a part thereof, other than as expressly stated herein with respect to the issuance 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 (the “DGCL”), and we express no opinion with respect to any other laws.

Subject to the foregoing and the other matters set forth herein, it is our opinion that, as of the date hereof, when the Shares shall have been duly registered on the books of the transfer agent and registrar therefor in the name or on behalf of the purchasers, and have been issued by the Company for legal consideration in excess of par value in the circumstances contemplated by the Plans, assuming in each case that the individual issuances, grants or awards under the Plans are duly authorized by all necessary corporate action of the Company and duly issued, granted or awarded and exercised in accordance with the requirements of law and the Plans (and the agreements and awards duly adopted thereunder and in accordance therewith), the issue and sale of the Shares will have been duly authorized by all necessary corporate action of the Company and the Shares will be validly issued, fully paid and nonassessable. In rendering the foregoing

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June 2, 2017 Page 2

opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL.

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 Securities Act. We consent to your filing this opinion as an exhibit to the Registration Statement. 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 Securities Act or the rules and regulations of the Commission thereunder.

Very truly yours,

/s/ Latham & Watkins LLP