Exhibit 5.1

March 23, 2015

RSP Permian, Inc.

3141 Hood Street, Suite 500

Dallas, Texas 75219

Ladies and Gentlemen:

We have acted as counsel for RSP Permian, Inc., a Delaware corporation (the “Company”), with respect to certain legal matters in connection with the registration by the Company under the Securities Act of 1933 (the “Securities Act”) of the offer and sale (the “Offering”) of 5,000,000 shares (the “Primary Shares”) of the Company’s common stock, par value $0.01 per share (the “Common Stock”), by the Company and 4,000,000 shares of Common Stock (the “Resale Shares” and, together with the Primary Shares, the “Shares”) by certain of the Company’s stockholders (the “Selling Stockholders”) pursuant to the Underwriting Agreement, dated March 17, 2015 (the “Underwriting Agreement”), by and among the Company, the Selling Stockholders and Barclays Capital Inc. (the “Underwriter”).

The Shares have been offered for sale pursuant to a prospectus supplement, dated March 17, 2015 (the “Prospectus Supplement”), and filed with the Securities and Exchange Commission (the “Commission”) pursuant to Rule 424(b) on March 19, 2015, to the prospectus (as amended and supplemented by the Prospectus Supplement, the “Prospectus”) that constitutes a part of the Company’s Registration Statement on Form S-3 (Registration No. 333-202823), filed with the Commission on March 17, 2015 (the “Registration Statement”), which Registration Statement became effective upon filing with the Commission pursuant to Rule 462(e) under the Securities Act.

In rendering the opinions set forth below, we have reviewed and relied upon (i) the Company’s Amended and Restated Certificate of Incorporation and Amended and Restated Bylaws, (ii) the Registration Statement and the Prospectus, (iii) resolutions of the Company’s board of directors and the pricing committee thereof relating to the Registration Statement and the Offering, (iv) the Underwriting Agreement, and (v) such other certificates, statutes and other instruments and documents as we considered necessary or appropriate for the purpose of rendering the opinions set forth below. In addition, we have reviewed such questions of law as we considered necessary or appropriate. As to matters of fact relevant to the opinions expressed below, and as to factual matters arising in connection with our review of corporate documents, records and other documents and writings, we have relied upon certificates and other communications of corporate officers of the Company, without further investigation as to the facts set forth therein.

For purposes of rendering the opinions set forth below, we have assumed that (i) all information contained in all documents we reviewed is true, correct and complete, (ii) all signatures on all documents we reviewed are genuine, (iii) all documents submitted to us as originals are true and complete, (iv) all documents submitted to us as copies are true and complete copies of the originals thereof, (v) all persons executing and delivering the documents

we examined were competent to execute and deliver such documents, (vi) all Shares will be issued and sold in compliance with applicable federal and state securities laws and in the manner stated in the Prospectus and the Registration Statement, and (vii) the Underwriting Agreement has been duly authorized and validly executed and delivered by the Underwriter.

Based upon and subject to the foregoing, and subject to the qualifications and limitations set forth herein, we are of the opinion that (1) the Primary Shares have been duly authorized and, when issued and paid for by the Underwriter as contemplated by the Underwriting Agreement, will be validly issued, fully paid and non-assessable, and (2) the Resale Shares have been duly authorized and are validly issued, fully paid and non-assessable:

Our opinions herein are limited in all respects to the General Corporation Law of the State of Delaware, which includes those statutory provisions as well as all applicable provisions of the Delaware Constitution and the reported judicial decisions interpreting such laws, and the federal laws of the United States of America, and we do not express any opinion as to the applicability of, or the effect thereon, of the laws of any other jurisdiction. We express no opinion as to any matter other than as set forth herein, and no opinion may be inferred or implied herefrom.  Our opinion is given as of the date hereof, and we undertake no, and hereby disclaim any, obligation to advise you of any change in any matter set forth herein.

We hereby consent to the filing of this opinion of counsel as Exhibit 5.1 to the Current Report on Form 8-K of the Company dated on or about the date hereof, to the incorporation by reference of this opinion of counsel into the Registration Statement and to the reference to our name under the caption “Legal Matters” in the Prospectus Supplement and the Prospectus. By giving such consent, we do not 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 issued thereunder.

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