Capstead Mortgage : Proxy Statement for October 15, 2021 Special Meeting

PROXY STATEMENT/PROSPECTUS

MERGER PROPOSED - YOUR VOTE IS VERY IMPORTANT

September 7, 2021

To the Stockholders of Capstead Mortgage Corporation:

The board of directors (the ''BSPRT Board'') of Benefit Street Partners Realty Trust, Inc. (''BSPRT'') and the board of directors (the ''Capstead Board'') of Capstead Mortgage Corporation (''Capstead''), each a Maryland corporation, each have approved an Agreement and Plan of Merger, dated as of July 25, 2021 (as such agreement may be amended or modified from time to time, the ''Merger Agreement''), by and among BSPRT, Rodeo Sub I, LLC, a Maryland limited liability company (''Merger Sub''), Capstead and Benefit Street Partners L.L.C., a Delaware limited liability company and the external advisor of BSPRT (the ''BSPRT Advisor'') pursuant to which Capstead will merge with and into Merger Sub, with Merger Sub continuing as the surviving company (the ''Merger''). The closing of the Merger will occur as promptly as practicable following satisfaction of all closing conditions set forth in the Merger Agreement, but either BSPRT or Capstead may terminate the Merger Agreement if the closing has not occurred by January 26, 2022. Upon completion of the Merger, BSPRT will change its name to ''Franklin BSP Realty Trust, Inc.'' and its shares of Class A common stock, par value $0.01 per share (''BSPRT Common Stock''), will be listed on the New York Stock Exchange under the symbol ''FBRT''.

Pursuant to the terms and subject to the conditions set forth in the Merger Agreement, at the effective time of the Merger, each issued and outstanding share of common stock, par value $0.01 per share, of Capstead (''Capstead Common Stock'') will be converted into the right to receive: from BSPRT, (A) a number of shares of BSPRT Common Stock equal to the quotient (rounded to the nearest one ten-thousandth) (the ''Exchange Ratio'') determined by dividing (i) Capstead's adjusted book value per share by (ii) BSPRT's adjusted book value per share (the ''Per Share Stock Consideration''), and (B) a cash amount equal to the product of (rounding to the nearest cent) (x) Capstead's adjusted book value per share multiplied by 15.75%, multiplied by (y) 22.5%, without any interest thereon (the ''Per Share Cash Consideration'' and together with the Per Share Stock Consideration, the ''Per Common Share BSPRT Consideration''); and from the BSPRT Advisor, a cash amount equal to the product of (rounding to the nearest cent) (A) Capstead's adjusted book value per share multiplied by 15.75%, multiplied by (B) 77.5%, without any interest thereon (the ''Advisor Cash Consideration'' and together with the Per Common Share BSPRT Consideration, the ''Total Per Common Share Consideration'').

Based on the number of shares of Capstead Common Stock and the number of Company performance units outstanding on the close of business on August 3, 2021, and an assumed Exchange Ratio of 0.3521 based on the adjusted book value per share of BSPRT Common Stock and Capstead Common Stock as of June 30, 2021, calculated in accordance with the Merger Agreement, we expect approximately 34,344,313 shares of BSPRT Common Stock will be issued in connection with the Merger. The actual Exchange Ratio will be publicly announced at least three business days before the Capstead special meeting of stockholders as described below.

Each share of Capstead's 7.50% Series E Cumulative Redeemable Preferred Stock, $0.10 par value per share, will be converted into the right to receive one share of a newly designated series of BSPRT preferred stock, par value $0.01 per share, which will be classified and designated as BSPRT's 7.50% Series E Cumulative Redeemable Preferred Stock (''BSPRT Series E Preferred Stock''), which will be listed on the New York Stock Exchange under the symbol ''FBRT PRE'' upon completion of the Merger.

Capstead will hold a special meeting of its stockholders, which will be held solely by means of remote communication live over the Internet on October 15, 2021, at 9:00 a.m., Central Time. Holders of Capstead Series E Preferred Stock will not be entitled to vote on any matter at the Capstead special meeting.

At the Capstead special meeting, the Capstead common stockholders will be asked to (i) consider and vote on a proposal (the ''Capstead Merger Proposal'') to approve the Merger and the other transactions contemplated by the Merger Agreement, (ii) consider and vote on a non-binding advisory proposal (the ''Capstead Non-Binding Compensation Advisory Proposal'') to approve the compensation that may be paid or become payable to Capstead's named executive officers that is based on or otherwise relates to the Merger and (iii) approve the adjournment of the Capstead special meeting, if necessary or appropriate, for the purpose of soliciting additional votes for the approval of the Capstead Merger Proposal (the ''Capstead Adjournment Proposal''). The Capstead Board has unanimously (i) determined that the Merger Agreement and the transactions contemplated thereby, including the Merger, are in the best interests of Capstead, (ii) approved the Merger Agreement and declared that the transactions contemplated by the Merger Agreement, including the Merger, are advisable, (iii) directed that the Merger and the other transactions contemplated by the Merger Agreement be submitted to the holders of Capstead Common Stock for consideration at the Capstead special meeting and

1. recommended, in accordance with and subject to the provisions of the Merger Agreement, that the holders of Capstead Common Stock approve the Merger and the other transactions contemplated by the Merger Agreement. The Capstead Board unanimously recommends that the Capstead common stockholders vote ''FOR'' the Capstead Merger Proposal, ''FOR'' the Capstead Non-BindingCompensation Advisory Proposal and ''FOR'' the Capstead Adjournment Proposal. Only those matters included in the notice of the Capstead special meeting (''Notice of Special Meeting'') may be considered and voted upon at the Capstead special meeting.

This proxy statement/prospectus provides detailed information about the Capstead special meeting, the Merger Agreement, the Merger and other related matters. A copy of the Merger Agreement is included as Annex A to this proxy statement/prospectus. We encourage you to read this proxy statement/prospectus, the Merger Agreement and the other annexes to this proxy statement/prospectus carefully and in their entirety. In particular, you should carefully consider the discussion in the section of this proxy statement/prospectus entitled ''Risk Factors'' beginning on page 26. You may also obtain more information about each company from the documents they file with the Securities and Exchange Commission (the ''SEC'').

Whether or not you plan to attend the Capstead special meeting virtually, please complete, date, sign and return, as promptly as possible, the enclosed proxy card in the accompanying reply envelope or authorize a proxy to vote your shares of Capstead Common Stock through the Internet or by telephone. You may also authorize a proxy to vote your shares over the Internet using the Internet address on the enclosed proxy card or by telephone using the toll-free number on the enclosed proxy card. If you authorize a proxy to vote your shares through the Internet or by telephone, you will be asked to provide the company number and control number from the enclosed proxy card. If you attend and vote at a special meeting virtually over the Internet, your vote by ballot will revoke any proxy previously submitted.

Your vote is very important, regardless of the number of shares of Capstead Common Stock you own. Whether or not you plan to attend the Capstead special meeting virtually, please authorize a proxy to vote your shares of stock as promptly as possible to make sure that your shares of stock are represented at the special meeting. Please note that the failure to vote, or authorize a proxy to vote, your shares of stock of Capstead is the equivalent of a vote against the Capstead Merger Proposal.

Thank you in advance for your continued support.

Sincerely,

Phillip A. Reinsch

President, Chief Executive Officer and Director

Capstead Mortgage Corporation

Neither the SEC nor any state securities regulatory agency has approved or disapproved of the securities to be issued in connection with the Merger or passed upon the adequacy or accuracy of this proxy statement/prospectus. Any representation to the contrary is a criminal offense.

This proxy statement/prospectus is dated September 7, 2021, and is first being mailed to the stockholders of Capstead on or about September 8, 2021.

CAPSTEAD MORTGAGE CORPORATION

8401 N. Central Expressway, Suite 800

Dallas, Texas 75225

[phone]

NOTICE OF SPECIAL MEETING OF CAPSTEAD COMMON STOCKHOLDERS

TO BE HELD ON OCTOBER 15, 2021

NOTICE IS HEREBY GIVEN that a special meeting of common stockholders (the ''Capstead special meeting'') of Capstead Mortgage Corporation, a Maryland corporation (''Capstead''), will be held solely by means of remote communication live over the Internet at www.proxydocs.com/CMO on October 15, 2021, at 9:00 a.m., Central Time, for the following purposes:

1. to consider and vote on a proposal (the ''Capstead Merger Proposal'') to approve the merger of Capstead with and into Rodeo Sub I, LLC, a Maryland limited liability company (''Merger Sub''), with Merger Sub continuing as the surviving entity, and the other transactions contemplated in connection therewith (collectively, the ''Merger''), pursuant to that certain Agreement and Plan of Merger, dated as of July 25, 2021 (as such agreement may be amended or modified from time to time, the ''Merger Agreement''), by and among Benefit Street Partners Realty Trust, Inc., a Maryland corporation (''BSPRT''), Merger Sub, Capstead and Benefit Street Partners L.L.C., a Delaware limited liability company and the external advisor of BSPRT (the ''BSPRT Advisor''), a copy of which is attached as Annex A to the proxy statement/prospectus accompanying this notice;
2. to consider and vote on a non-binding advisory proposal to approve the compensation that may be paid or become payable to Capstead's named executive officers that is based on or otherwise relates to the Merger (the ''Capstead Non-Binding Compensation Advisory Proposal''); and
3. to consider and vote on a proposal to approve the adjournment of the Capstead special meeting, if necessary or appropriate, for the purpose of soliciting additional votes for the approval of the Capstead Merger Proposal.

Capstead will transact no other business at the Capstead special meeting or any postponement or adjournment thereof. Please refer to the attached proxy statement/prospectus for further information with respect to the business to be transacted at the Capstead special meeting. The board of directors of Capstead (the ''Capstead Board'') has fixed the close of business on August 26, 2021 as the record date (the ''Capstead Record Date'') for the determination of the holders of shares of common stock, par value $0.01 per share, of Capstead (the ''Capstead Common Stock'') entitled to the notice of, and to vote at, the Capstead special meeting and any postponement or adjournment thereof. Accordingly, only Capstead common stockholders at the close of business on the Capstead Record Date are entitled to the notice of, and to vote at, the Capstead special meeting and any postponement or adjournment thereof.

The Capstead Board has unanimously (i) determined that the Merger Agreement and the transactions contemplated thereby, including the Merger, are in the best interests of Capstead, (ii) approved the Merger Agreement and declared that the transactions contemplated by the Merger Agreement, including the Merger, are advisable,

1. directed that the Merger and the other transactions contemplated by the Merger Agreement be submitted to the holders of Capstead Common Stock for consideration at the Capstead special meeting and (iv) recommended, in accordance with and subject to the provisions of the Merger Agreement, that the holders of Capstead Common Stock approve the Merger and the other transactions contemplated by the Merger Agreement. The Capstead Board unanimously recommends that the Capstead common stockholders vote ''FOR'' the Capstead Merger Proposal, ''FOR'' the Capstead Non-BindingCompensation Advisory Proposal and ''FOR'' the Capstead Adjournment Proposal.

Your vote is very important, regardless of the number of shares of Capstead Common Stock you own. Whether or not you plan to attend the Capstead special meeting virtually, please authorize a proxy to vote your shares of Capstead Common Stock as promptly as possible to make sure that your shares are represented at the Capstead special meeting. Properly executed proxy cards with no instructions indicated on the proxy card will be voted ''FOR'' the Capstead Merger Proposal, ''FOR'' the Capstead Non-Binding Compensation Advisory Proposal and ''FOR'' the Capstead Adjournment Proposal.

To ensure your representation at the Capstead special meeting, you are urged to vote your shares of Capstead Common Stock (1) by phone, (2) via the Internet before the meeting or during the meeting or (3) by marking, signing,

dating and promptly returning the proxy card in the enclosed postage-paid envelope for that purpose. Whether or not you plan to attend the virtual Capstead special meeting, we urge you to vote in advance of the Capstead special meeting by one of the methods described above. Any Capstead stockholder attending the virtual Capstead special meeting may vote at the Capstead special meeting even if he or she previously submitted a proxy. If your shares of Capstead Common Stock are held by a bank, broker or other agent, please follow the instructions from your bank, broker or other agent to have your shares voted.

If you do not vote on the Capstead Merger Proposal, this will have the same effect as a vote by you against the approval of the Capstead Merger Proposal.

Please note that if you hold shares of Capstead Common Stock in different accounts, it is important that you vote or authorize a proxy to vote the shares of Capstead Common Stock represented by each account. If you attend the Capstead special meeting virtually over the Internet, you may revoke your proxy and vote electronically at the Capstead special meeting, even if you have previously returned your proxy card or authorized, through the Internet or by telephone, a proxy to vote your shares of Capstead Common Stock.

Only stockholders of record and beneficial owners of shares of Capstead Common Stock as of the close of business on the Capstead Record Date may attend and participate in the Capstead special meeting, including voting and asking questions during the virtual Capstead special meeting. You will not be able to attend the Capstead special meeting physically in person.

In order to attend the Capstead special meeting, you must register at www.proxydocs.com/CMO by 5:00 p.m., Eastern Time on October 13, 2021. Upon completing your registration, you will receive further instructions via email, including a unique link that will allow you access to the Capstead special meeting and to vote and submit questions during the Capstead special meeting.

As part of the registration process, you must enter the control number located on your proxy card or in the instructions that accompanied your proxy materials. If you are a beneficial owner of shares registered in the name of a broker, bank or other nominee, you will also need to provide the registered name on your account and the name of your broker, bank or other nominee as part of the registration process.

On the day of the Capstead special meeting, October 15, 2021, stockholders may begin to log in to the virtual-only Capstead special meeting 15 minutes prior to the Capstead special meeting. The Capstead special meeting will begin promptly at 9:00 a.m., Central Time. Please allow ample time for online login.

We will have technicians ready to assist you with any technical difficulties you may have accessing the Capstead special meeting. If you encounter any difficulties accessing the virtual-only Capstead special meeting platform, including any difficulties voting or submitting questions, you may call the technical support number that will be posted in your instructional email.

This notice and the enclosed proxy statement/prospectus are first being mailed to Capstead stockholders on or about September 8, 2021.

By Order of the Board of Directors,

Lance J. Phillips

Senior Vice President, Chief Financial Officer and

Secretary

Dallas, Texas

September 7, 2021

ADDITIONAL INFORMATION

This proxy statement/prospectus incorporates important business and financial information about BSPRT and Capstead from other documents that are not included in or delivered with this proxy statement/prospectus. This information is available to you without charge upon your request. To obtain timely delivery, you must request the information no later than five business days before the date of the Capstead special meeting. You can obtain copies of this proxy statement/prospectus and the documents incorporated by reference into this proxy statement/prospectus by requesting them from:

Capstead Mortgage Corporation

8401 North Central Expressway, Suite 800

Dallas, Texas 75225

1. 874-2339
   Attention: Investor Relations

or

Georgeson LLC

1290 Avenue of the Americas, 9th Floor

New York, New York 10104

[phone]

Investors may also consult BSPRT's or Capstead's website for more information concerning the Merger and other related transactions described in this proxy statement/prospectus. BSPRT's website is www.bsprealtytrust.com. Capstead's website is www.capstead.com. Each company's public filings are also available at www.sec.gov. The information contained on BSPRT's and Capstead's websites is not part of this proxy statement/prospectus and is not incorporated herein by reference. The references to BSPRT's and Capstead's websites are intended to be inactive textual references only.

If you would like to request copies of this proxy statement/prospectus and any documents that are incorporated by reference into this proxy statement/prospectus, please do so by October 8, 2021, in order to receive them before the Capstead special meeting.

For more information, see ''Where You Can Find More Information and Incorporation by Reference'' beginning on page 194.

ABOUT THIS DOCUMENT

This proxy statement/prospectus, which forms part of a registration statement on Form S-4 (Registration Statement No. 333-258947) filed by BSPRT with the SEC, constitutes a prospectus of BSPRT for purposes of the Securities Act of 1933, as amended (the ''Securities Act''), with respect to (i) the shares of BSPRT Common Stock to be issued to Capstead common stockholders in exchange for shares of Capstead Common Stock and (ii) the shares of BSPRT Series E Preferred Stock to be issued to holders of Capstead Series E Preferred Stock, in each case pursuant to the Merger Agreement. This proxy statement/prospectus also constitutes a proxy statement for Capstead for purposes of the Securities Exchange Act of 1934, as amended (the ''Exchange Act''). In addition, it constitutes a notice of special meeting with respect to the Capstead special meeting.

No one has been authorized to provide you with information that is different from that contained in, or incorporated by reference into, this proxy statement/prospectus. This proxy statement/prospectus is dated September 7, 2021, and you should not assume that the information contained in, or incorporated by reference into, this proxy statement/prospectus is accurate as of any date other than that date (or, in the case of documents incorporated by reference, their respective dates). Neither the mailing of this proxy statement/prospectus to Capstead stockholders nor the BSPRT Common Stock Issuance to Capstead common stockholders in the Merger pursuant to the Merger Agreement will create any implication to the contrary.

This proxy statement/prospectus does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction in which or to any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Information contained in this proxy statement/prospectus regarding BSPRT has been provided by BSPRT and information contained in or incorporated by reference into this proxy statement/prospectus regarding Capstead has been provided by Capstead. BSPRT and Capstead have both contributed to the information relating to the Merger contained in this proxy statement/prospectus.