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Exhibit 5.1

May 6, 2021

NextCure, Inc.

9000 Virginia Manor Road

Suite 200

Beltsville, Maryland 20705

Re:       Registration Statement on Form S-3

Ladies and Gentlemen:

We refer to the Registration Statement on Form S-3, File No. 333-241706, as amended by Post-Effective Amendments Nos.1 and 2 to Form S-3 (as so amended, the “Registration Statement”), filed by NextCure, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission (the “Commission”), which Registration Statement became effective on March 12, 2021 under the Securities Act of 1933, as amended (the “Securities Act”). Pursuant to the Registration Statement, the Company may from time to time issue up to an aggregate offering price of $75,000,000 of shares (the “Shares”) of its Common Stock, $0.001 par value per share (the “Common Stock”). The Shares are to be sold by the Company pursuant to a sales agreement, dated May 6, 2021 (the “Sales Agreement”) between the Company and SVB Leerink LLC.

This opinion letter is being delivered in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act.

We have examined the Registration Statement, the Company’s base prospectus dated March 12, 2021 (the “Base Prospectus”), the Company’s prospectus supplement dated May 6, 2021 supplementing the Base Prospectus and relating to the Shares (the “Prospectus Supplement”) the Sales Agreement, the Company’s certificate of incorporation (as amended and restated, the “Certificate of Incorporation”), the Bylaws of the Company, as amended to the date hereof (the “Bylaws”) and the resolutions (the “Resolutions”) adopted by the board of directors of the Company (the “Board”) and the Audit Committee of the Board, relating to the Registration Statement and the Sales Agreement. We have also examined originals, or copies of originals certified to our satisfaction, of such agreements, documents, certificates and statements of the Company and other corporate documents and instruments, and have examined such questions of law, as we have considered relevant and necessary as a basis for this opinion letter. We have assumed the authenticity of all documents submitted to us as originals, the genuineness of all signatures, the legal capacity of all persons and the conformity with the original documents of any copies thereof submitted to us for examination. As to facts relevant to the opinions expressed herein, we have relied without independent investigation or verification upon, and assumed the accuracy and completeness of, certificates, letters and oral and written statements and representations of public officials and officers and other representatives of the Company.

Sidley Austin (DC) LLP is a Delaware limited liability partnership doing business as Sidley Austin LLP and practicing in affiliation with other Sidley Austin partnerships.

May 6, 2021
Page 2

Based on the foregoing, we are of the opinion that the Shares will be validly issued, fully paid and non-assessable when: (i) the Board or a duly authorized committee thereof shall have duly adopted final resolutions in conformity with the Certificate of Incorporation, the Bylaws and the Resolutions, setting the price of the Shares, and authorizing the issue and sale of the Shares, and (ii) certificates representing such Shares shall have been duly executed, countersigned and registered and duly delivered to the purchasers thereof against payment of the agreed consideration therefor in an amount not less than the par value thereof or, if any such Shares are to be issued in uncertificated form, the Company’s books shall reflect the issuance of such Shares to the purchasers thereof against payment of the agreed consideration therefor in an amount not less than the par value thereof, all in accordance with the Sales Agreement.

For the purposes of this opinion letter, we have assumed that, at the time of the issuance, sale and delivery of Shares:

This opinion letter is limited to the General Corporation Law of the State of Delaware. We express no opinion as to the laws, rules or regulations of any other jurisdiction, including, without limitation, the federal laws of the United States of America or any state securities or blue sky laws.

We hereby consent to the filing of this opinion letter as an Exhibit to the Company’s Current Report on Form 8-K to be filed with the Commission on or after the date hereof, which Current Report on Form 8-K will be incorporated by reference into the Registration Statement, and to all references to our Firm under the caption “Legal Matters” in the Prospectus Supplement. 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 Securities Act.