Exhibit 5.1

March 27, 2015

Aldeyra Therapeutics, Inc.

131 Hartwell Avenue, Suite 320

Lexington, MA 02421

Ladies and Gentlemen:

You have requested our opinion with respect to certain matters in connection with the filing by Aldeyra Therapeutics, Inc. (the “Company”) of a registration statement on Form S-1 (the “Registration Statement”) with the Securities and Exchange Commission under the Securities Act of 1933, as amended, relating to the sale by a certain selling stockholder of an aggregate of up to 423,056 shares of the Company’s Common Stock, which includes 211,528 shares of the Company’s Common Stock (the “Shares”) that are issued and outstanding and 211,528 shares of Common Stock issuable upon the exercise of a warrant (the “Warrant”) that is issued and outstanding. The Shares and the shares of the Company’s Common Stock issuable upon exercise of the Warrant (the “Warrant Shares”) are to be sold by a certain selling stockholder of the Company in the manner contemplated by the prospectus contained in the Registration Statement.

In connection with this opinion, we have examined the Registration Statement and such other documents, records, certificates, memoranda and other instruments as we deem necessary as a basis for this opinion. We have assumed the genuineness of all signatures, the legal capacity of natural persons, the genuineness and authenticity of all documents submitted to us as originals, the conformity to originals of all documents submitted to us as copies thereof and the due execution and delivery of all documents where due execution and delivery are a prerequisite to the effectiveness thereof. As to questions of fact material to this opinion, we have relied upon certificates or comparable documents of public officials and of officers and representatives of the Company.

Based on the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that the Shares are validly issued, fully paid and non-assessable and that the Warrant Shares have been duly authorized for issuance and, when issued and exercised in accordance with the Warrant, the Registration Statement and the related prospectus and prospectus supplement, including the payment of the exercise price therefor, the Warrant Shares will be validly issued, fully paid and nonassessable.

We do not express any opinion herein concerning any law other than the Delaware General Corporation Law (including the statutory provisions, all applicable provisions of the Delaware Constitution and reported judicial decisions interpreting the foregoing).

This opinion letter is rendered to you in connection with the preparation and filing of the Registration Statement. This opinion letter may not be relied upon by you for any other purpose without our prior written consent. This opinion letter speaks only as of the date hereof. We expressly disclaim any responsibility to advise you of any development or circumstance of any kind, including any change of law or fact, that may occur after the date of this opinion letter that might affect the opinions expressed therein.

We hereby consent to the filing 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 and in the Prospectus included therein. In giving this 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 of the Commission promulgated thereunder.

Very truly yours,

/s/ Gunderson Dettmer Stough Villeneuve Franklin & Hachigian, LLP