Exhibit 5.1

CBTX, Inc.
9 Greenway Plaza, Suite 110
Houston, Texas 77046

Ladies and Gentlemen:

We have acted as special counsel to CBTX, Inc., a Texas corporation (the “Company”), in connection with the registration under the Securities Act of 1933, as amended (the “Securities Act”), of 2,760,000 shares of the Company’s common stock, par value $0.01 per share (the “Shares”), as described in the Company’s Registration Statement on Form S-1 (File No. 333-220930) initially filed with the U.S. Securities and Exchange Commission with respect to the Shares on October 13, 2017 (as amended and as may subsequently be amended, the “Registration Statement”). The Shares to be issued and sold by the Company include up to 360,000 Shares which may be sold by the Company at the option of the Underwriters (as defined below), if any, pursuant to the Registration Statement.

We understand that the Shares are to be sold to the underwriters for resale to the public as described in the Registration Statement and pursuant to that certain underwriting agreement to be entered into by and among the Company, Stephens Inc., and Keefe, Bruyette & Woods, Inc., as representatives of the several underwriters (the “Underwriters”) named in Annex A thereto (the “Underwriting Agreement”).

In rendering the opinions set forth below, we have reviewed and relied upon: (i) the Registration Statement; (ii) the Company’s Amended and Restated Certificate of Formation and Second Amended and Restated Bylaws; (iii) resolutions of the Company’s Board of Directors relating to the Registration Statement; (iv) the Underwriting Agreement; and (v) such other certificates, statutes and other instruments and documents as we considered necessary or appropriate for purposes of rendering the opinions expressed in this letter.  In addition, we have reviewed such questions of law as we considered necessary or appropriate to enable us to render the opinions expressed in this letter.  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 officers and employees of the Company without further investigation as to the facts set forth in such certificates and communications.

In connection with rendering the opinions expressed in this letter, we have assumed that: (i) the documents reviewed and relied upon in giving the opinions are true and correct copies of the original documents, and the signatures on such documents are genuine; (ii) the representations of officers and employees of the Company are correct as to questions of fact; (iii) the persons identified as officers of the Company are actually serving as such and that any certificates representing the Shares will be properly

Norton Rose Fulbright US LLP is a limited liability partnership registered under the laws of Texas.

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executed by one or more such persons; (iv) the persons executing the documents examined by us have the legal capacity to execute such documents; (v) all Shares will be offered and sold in compliance with the manner specified in the Registration Statement and the prospectus forming a part of the Registration Statement; and (vi) a pricing committee of the board of directors of the Company will have taken action necessary to set the sale price of the Shares.  In addition, we have assumed that the Registration Statement, and any amendments thereto (including post-effective amendments), will have become effective under the Securities Act and a prospectus will have been filed with the Securities and Exchange Commission describing the pricing of the Shares offered thereby.

Based on the foregoing, and subject to the assumptions, qualifications, limitations and exceptions set forth in this letter, we are of the opinion that the Shares have been duly authorized and, when issued and paid for by the Underwriters as contemplated by the Underwriting Agreement, will be validly issued, fully paid and non-assessable.

The foregoing opinions are limited in all respects to the to the applicable provisions of the laws of the State of Texas and the applicable federal laws of the United States of America, and we do not express any opinion as to the applicability or effect of the laws of any other jurisdiction.  We express no opinion as to any matter other than as set forth in this letter, and no other opinion may be inferred or implied.  Our opinion is given as of the date of this letter, and we undertake no, and disclaim any, obligation to advise you of any change in any matter set forth in this letter.

We hereby consent to the reference to our firm under the caption “Legal Matters” in the prospectus forming a part of the Registration Statement and to the filing of this opinion as an exhibit to the Registration Statement. 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 thereunder.

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