Exhibit 5.1

[AKIN GUMP STRAUSS HAUER & FELD LLP LETTERHEAD]

June 18, 2013

Diamondback Energy, Inc.

500 West Texas

Suite 1225

Midland, Texas 79701

Ladies and Gentlemen:

We have acted as special counsel to Diamondback Energy, Inc., a Delaware corporation (the “Company”), in connection with the registration of shares of the Company’s common stock, par value $0.01 per share (“Common Stock”), pursuant to a registration statement on Form S-1 under the Securities Act of 1933, as amended (the “Act”), filed by the Company with the Securities and Exchange Commission (the “Commission”) on June 7, 2013 (File No. 333-189176) (as amended, the “Initial Registration Statement”) and a registration statement relating to the Initial Registration Statement filed pursuant to Rule 462(b) promulgated under the Act (the “462(b) Registration Statement”). The 462(b) Registration Statement relates to the registration of 1,119,222 shares of Common Stock (the “Additional Shares”), all of which will be sold by the selling stockholders listed in the Initial Registration Statement (the “Selling Stockholders”), including 119,222 shares that may be purchased by the underwriters pursuant to an option to purchase additional shares from the Selling Stockholders under the terms of an underwriting agreement dated June 18, 2013 by and among the Company, the Selling Stockholders and Credit Suisse Securities (USA) LLC, as representative of the several underwriters named therein (the “Underwriting Agreement”).

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 authenticity of all documents submitted to us as originals, the conformity to authentic original documents of all copies submitted to us as conformed and certified or reproduced copies.

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

Diamondback Energy, Inc.

June 18, 2013

Page 2

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

We hereby consent to the filing of this letter as an exhibit to the 462(b) Registration Statement and to the use of our name in the related prospectus 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.