Exhibit 5.1

Oncternal Therapeutics, Inc.

12230 El Camino Real, Suite 300

San Diego, CA 92130

Ladies and Gentlemen:

We have acted as special counsel to Oncternal Therapeutics, Inc., a Delaware corporation (the “Company”), in connection with the proposed issuance of up to 2,581,867 shares (the “Shares”) of common stock of the Company, par value $0.001 per share (the “Common Stock”). The Shares are included in a registration statement on Form S-3 under the Securities Act of 1933, as amended (the “Act”), filed with the Securities and Exchange Commission (the “Commission”) on December 22, 2017 (Registration No. 333-222268) (as so filed and amended, the “Registration Statement”), a base prospectus dated January 5, 2018 included in the Registration Statement at the time it originally became effective (the “Base Prospectus”) and a prospectus supplement dated July 17, 2020 filed with the Commission pursuant to Rule 424(b) of the Act (together with the Base Prospectus, the “Prospectus”). The Shares are being sold pursuant to a securities purchase agreement, dated as of July 17, 2020, by and between the Company and the investors named therein (the “Securities Purchase Agreement”).

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 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

July 21, 2020

Page 2

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 against payment therefor (not less than par value) in the circumstances contemplated by the Securities Purchase Agreement, 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 opinion, we have assumed that the Company will comply with all applicable notice requirements regarding uncertificated shares provided in the DGCL.

We bring your attention to the fact that Latham & Watkins LLP attorneys rendering services in connection with the offering and investment funds affiliated with the firm own certain securities of the Company.

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 Company’s Current Report on Form 8-K dated July 21, 2020 and to the reference to our firm in the Prospectus under the heading “Legal Matters.” 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.