Exhibit 5.1

[AKIN, GUMP, STRAUSS, HAUER & FELD, L.L.P. LETTERHEAD]

June 17, 2013

Diamondback Energy, Inc.

500 West Texas, Suite 1225

Midland, Texas 79701

Registration Statement on Form S-1

File No. 333-189176

Ladies and Gentlemen:

We have acted as special counsel to Diamondback Energy, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing by the Company with the Securities and Exchange Commission of a Registration Statement on Form S-1, as amended (File No. 333-189176) (the “Registration Statement”), under the Securities Act of 1933, as amended (the “Act”). The Registration Statement relates to an underwritten public offering by the selling stockholders listed in the Registration Statement (the “Selling Stockholders”) of up to 5,750,000 shares (including up to 750,000 shares subject to the Underwriters’ (as defined below) overallotment option) (the “Shares”) of the Company’s common stock, par value $0.01 per share (“Common Stock”), pursuant to the terms of an underwriting agreement to be executed by the Company, the Selling Stockholders and Credit Suisse Securities (USA) LLC, as representative of the underwriters named therein (the “Underwriters”). This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Act.

We have examined originals or certified copies of such corporate records of the Company and other certificates and documents of officials of the Company, public officials and others as we have deemed appropriate for purposes of this letter. We have assumed the genuineness of all signatures, the legal capacity of all natural persons, the authenticity of all documents submitted to us as originals and the conformity to authentic original documents of all copies submitted to us as conformed, certified or reproduced copies. We have also assumed that, upon sale and delivery, the certificates for the Shares, if certificated, will conform to the specimen thereof filed as an exhibit to the Registration Statement and will have been duly countersigned by the transfer agent and duly registered by the registrar for the Common Stock or, if uncertificated, valid book-entry notations for the transfer of the Shares in uncertificated form will have been duly made in the share register of the Company. As to various questions of fact relevant to this letter, we have relied, without independent investigation, upon certificates of public officials and certificates of officers of the Company, all of which we assume to be true, correct and complete.

Diamondback Energy, Inc.

June 17, 2013

Page 2

Based upon the foregoing, and subject to the assumptions, exceptions, qualifications and limitations stated herein, we are of the opinion that the Shares have been duly authorized and validly issued and are fully paid and non-assessable.

The opinion and other matters in this letter are qualified in their entirety and subject to the following:

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the use of our name in the Prospectus forming a part of the Registration Statement under the caption “Legal Matters”. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Act and the rules and regulations thereunder.