Exhibit 5.1

Gibson, Dunn & Crutcher LLP   811 Main Street Houston, TX 77002-6117 Tel 346.718.6600 www.gibsondunn.com

March 22, 2019

U.S. Well Services, Inc.

1360 Post Oak Boulevard, Suite 1800

Houston, Texas 77056

Re: U.S. Well Services, Inc.

Registration Statement on Form S-3

Ladies and Gentlemen:

We have acted as counsel to U.S. Well Services, Inc., a Delaware corporation (the “Company”), in connection with the preparation and filing with the U.S. Securities and Exchange Commission (the “Commission”) of a Registration Statement on Form S-3 (the “Registration Statement”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the registration under the Securities Act and the proposed issuance and sale from time to time pursuant to Rule 415 under the Securities Act, together or separately and in one or more series (if applicable) of:

(i) shares of the Company’s Class A common stock, par value $0.0001 per share (the “Common Stock”); and

(ii) shares of the Company’s preferred stock, par value $0.0001 per share (the “Preferred Stock”).

The Common Stock and Preferred Stock are collectively referred to herein as the “Securities.”

In arriving at the opinions expressed below, we have examined originals, or copies certified or otherwise identified to our satisfaction as being true and complete copies of the originals, of such documents, corporate records, certificates of officers of the Company and of public officials and other instruments as we have deemed necessary or advisable to enable us to render these opinions. In our examination, we have assumed the genuineness of all signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals and the conformity to original documents of all documents submitted to us as copies. As to any facts material to these opinions, we have relied to the extent we deemed appropriate and without independent investigation upon statements and representations of officers and other representatives of the Company and others.

Beijing ● Brussels ● Century City ● Dallas ● Denver ● Dubai ● Frankfurt ● Hong Kong ● Houston ● London ● Los Angeles ● Munich

New York ● Orange County ● Palo Alto ● Paris ● San Francisco ● São Paulo ● Singapore ● Washington, D.C.

U.S. Well Services, Inc.

March 22, 2019

Page 2

We have assumed without independent investigation that:

(i) at the time any Securities are sold pursuant to the Registration Statement (the “Relevant Time”), the Registration Statement and any supplements and amendments thereto (including post-effective amendments) will be effective and will comply with all applicable laws;

(ii) at the Relevant Time, a prospectus supplement will have been prepared and filed with the Commission describing the Securities offered thereby and all related documentation and will comply with all applicable laws;

(iii) all Securities will be issued and sold in the manner stated in the Registration Statement and the applicable prospectus supplement;

(iv) at the Relevant Time, all corporate or other action required to be taken by the Company to duly authorize each proposed issuance of Securities and any related documentation (including (i) the due reservation of any shares of Common Stock for issuance upon exercise, conversion or exchange of any Preferred Stock (a “Convertible Security”), and (ii) the execution (in the case of certificated Securities), delivery and performance of the Securities and any related documentation referred to in paragraphs 1 and 2 below) shall have been duly completed and shall remain in full force and effect;

(v) upon issuance of any Common Stock, including upon exercise, conversion or exchange of any Convertible Security, or Preferred Stock, the total number of shares of Common Stock or Preferred Stock issued and outstanding will not exceed the total number of shares of Common Stock or Preferred Stock, as applicable, that the Company is then authorized to issue under its certificate of incorporation and other relevant documents; and

(vi) at the Relevant Time, a definitive purchase, underwriting or similar agreement and any other necessary agreement with respect to any Securities offered or issued will have been duly authorized by all necessary corporate or other action of the Company and duly executed and delivered by the Company and the other parties thereto.

U.S. Well Services, Inc.

March 22, 2019

Page 3

Based on the foregoing and in reliance thereon, and subject to the assumptions, exceptions, qualifications and limitations set forth herein, we are of the opinion that:

1. With respect to shares of Common Stock, when:

a. such shares of Common Stock have been duly executed (in the case of certificated shares) and delivered either (i) in accordance with the applicable definitive purchase, underwriting or similar agreement for the consideration provided for therein, or (ii) upon conversion, exercise or exchange of any Convertible Security, in accordance with the terms of such Convertible Security or the instrument governing such Convertible Security providing for such conversion, exercise or exchange, and for any additional consideration specified therein, which consideration (including any consideration paid for such Convertible Security), on a per-share basis, shall in either event not be less than the par value of the Common Stock, and

b. any such Convertible Security was previously validly issued and is fully paid and non-assessable,

such shares of Common Stock will be validly issued, fully paid and non-assessable.

2. With respect to any shares of Preferred Stock, when:

a. the certificate of designations relating to such Preferred Stock (the “Certificate of Designations”) has been duly executed and filed with the Office of the Secretary of State of the State of Delaware, and

b. such shares have been issued in accordance with the applicable definitive purchase, underwriting or similar agreement and for the consideration therefor provided for therein,

such shares of Preferred Stock will be validly issued, fully paid and non-assessable.

U.S. Well Services, Inc.

March 22, 2019

Page 4

We render no opinion herein as to matters involving the laws of any jurisdiction other than the United States of America and the Delaware General Corporation Law (the “DGCL”). We are not admitted to practice in the State of Delaware; however, we are generally familiar with the DGCL as currently in effect and have made such inquiries as we consider necessary to render the opinions contained in paragraphs 1 and 2 above. This opinion is limited to the effect of the current state of the laws of the United States of America and, to the limited extent set forth above, the laws of the State of Delaware and the facts as they currently exist. We assume no obligation to revise or supplement this opinion in the event of future changes in such laws or the interpretations thereof or such facts.

You have informed us that you intend to issue Securities from time to time on a delayed or continuous basis, and we understand that prior to issuing any Securities pursuant to the Registration Statement (i) you will advise us in writing of the terms thereof, and (ii) you will afford us an opportunity to (x) review the operative documents pursuant to which such Securities are to be issued or sold (including the applicable offering documents), and (y) file such supplement or amendment to this opinion (if any) as we may consider necessary or appropriate.

We consent to the filing of this opinion as an exhibit to the Registration Statement, and we further consent to the use of our name under the caption “Legal Matters” in the Registration Statement and the prospectus that forms a part thereof. In giving these consents, we do not thereby 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/ Gibson, Dunn & Crutcher LLP