Exhibit 5.1

Simpson Thacher & Bartlett llp
2475 hanover street palo alto, ca 94304
telephone: +1-650-251-5000 facsimile: +1-650-251-5002
Direct Dial Number	E-mail Address

, 2023

Waystar Holding Corp.

1550 Digital Drive, #300

Lehi, Utah 84043

Ladies and Gentlemen:

We have acted as counsel to Waystar Holding Corp., a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-1 (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Act”), relating to the issuance by the Company of an aggregate of shares of common stock, par value $0.01 per share (the “Common Stock”) (together with any additional shares of such stock that may be issued by the Company pursuant to Rule 462(b) (as prescribed by the Commission pursuant to the Act) in connection with the offering described in the Registration Statement, the “Shares”).

We have examined the Registration Statement and a form of the Amended and Restated Certificate of Incorporation of the Company (the “Amended Charter”), which has been filed with the Commission as an exhibit to the Registration Statement. In addition, we have examined, and have relied as to matters of fact upon, originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and such certificates or comparable documents of public officials and of officers and representatives of the Company and have made such other investigations as we have deemed relevant and necessary in connection with the opinion hereinafter set forth.

BEIJING Brussels HONG KONG Houston LONDON Los Angeles New York SÃO PAULO TOKYO Washington, D.C.

Waystar Holding Corp. -2- , 2023

In rendering the opinion set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that, (A) when the Amended Charter has been duly filed with the Secretary of State of the State of Delaware, (B) when the Board of Directors of the Company (the “Board”) has taken all necessary corporate action to authorize and approve the sale price of the Shares and (C) upon payment and delivery in accordance with the applicable definitive underwriting agreement approved by the Board, the Shares will be validly issued, fully paid and nonassessable.

We do not express any opinion herein concerning any law other than the Delaware General Corporation Law.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption “Legal matters” in the prospectus included in the Registration Statement.

Very truly yours,

SIMPSON THACHER & BARTLETT LLP