UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 7)*

Blackstone Mortgage Trust, Inc.

(Name of Issuer)

Class A Common Stock, par value $0.01 per share

(Title of Class of Securities)

09257W100

(CUSIP Number)

Randall S. Rothschild

The Blackstone Group L.P.

345 Park Avenue

New York, New York 10154

Tel: (212) 583-5000

with a copy to:

Brian M. Stadler, Esq.

Andrew R. Keller, Esq.

Simpson Thacher & Bartlett LLP

425 Lexington Avenue

New York, NY 10017

Tel: (212) 455-2000

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

April 13, 2015

(Date of Event Which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of §§240.13d-1(e), 240.13d-1(f) or 240.13d-1(g), check the following box.  ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See §240.13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

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This Amendment No. 7 (“Amendment No. 7”) to Schedule 13D relates to the class A common stock, par value $0.01 per share (“Common Stock”), of Blackstone Mortgage Trust, Inc., a Maryland corporation (the “Issuer”), and amends and supplements the initial statement on Schedule 13D filed on December 21, 2012, as amended and supplemented by Amendment No. 1 to Schedule 13D filed on March 27, 2013, Amendment No. 2 to Schedule 13D filed on May 8, 2013, Amendment No. 3 to Schedule 13D filed on May 20, 2013, Amendment No. 4 to Schedule 13D filed on May 31, 2013, Amendment No. 5 to Schedule 13D filed on October 11, 2013 and Amendment No. 6 to Schedule 13D filed on January 10, 2014 (collectively, the “Schedule 13D”).

Capitalized terms used but not defined in this Amendment No. 7 shall have the same meanings ascribed to them in the Schedule 13D.

Item 4 of the Schedule 13D is hereby supplemented as follows:

On April 13, 2015, the Issuer commenced a public offering of shares of its Common Stock (the “Proposed 2015 Offering”) under its Registration Statement on Form S-3 (File No. 333-190191) filed with the Securities and Exchange Commission. In connection with the Proposed Offering, Blackstone Treasury Holdings III and certain other affiliates of Blackstone have communicated an interest in purchasing an aggregate of up to $37.5 million of shares of the Issuer’s Common Stock in the Proposed 2015 Offering at the public offering price. However, because indications of interest are not binding agreements or commitments to purchase and no price has been fixed, the underwriters for the Proposed 2015 Offering could determine to sell more, less or no shares of Common Stock to the Reporting Persons and the Reporting Persons may purchase more, less or no shares of Common Stock based on their continuing review of the Reporting Persons’ investments in the Issuer.

Item 5 of the Schedule 13D is hereby amended and restated as follows:

(a) – (b) As of the date hereof, Blackstone Treasury Holdings III directly holds 2,460,784 shares of Common Stock, the Manager directly holds 677,372 shares of Common Stock and Mr. Schwarzman directly holds 44,452 shares of Common Stock.

Blackstone Holdings III is the sole member of Blackstone Treasury Holdings III. Blackstone Holdings III GP L.P. is the general partner of Blackstone Holdings III. Blackstone Holdings III GP Management L.L.C. is the general partner of Blackstone Holdings III GP L.P. Blackstone is the managing member of Blackstone Holdings III GP Management L.L.C. Blackstone Group Management L.L.C. is the general partner of The Blackstone Group L.P. Blackstone Group Management L.L.C. is wholly owned by its senior managing directors and controlled by its founder, Stephen A. Schwarzman. The Manager is an indirect subsidiary of Blackstone.

The aggregate number and percentage of the shares of Common Stock beneficially owned by each Reporting Person and, for each Reporting Person, the number of shares as to which there is sole power to vote or to direct the vote, shared power to vote or to direct the vote,

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sole power to dispose or to direct the disposition, or shared power to dispose or to direct the disposition are set forth on rows 7 through 11 and row 13 of the cover pages of this Schedule 13D and are incorporated herein by reference.

The percentage of shares of Common Stock beneficially owned by each Reporting Person set forth on the cover pages of this Schedule 13D is calculated based on 58,451,077 shares of Common Stock outstanding as of April 10, 2015, as reported in a preliminary prospectus supplement dated April 13, 2015 of the Issuer, filed on April 13, 2015.

Each of the Blackstone Entities and Mr. Schwarzman may be deemed to beneficially own the shares of Common Stock beneficially owned by Blackstone Treasury Holdings III and each of Blackstone, Blackstone Group Management L.L.C. and Mr. Schwarzman may be deemed to beneficially own the shares of Common Stock beneficially owned by the Manager.

Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission that any of the Reporting Persons (other than Blackstone Treasury Holdings III and Mr. Schwarzman, to the extent they directly hold shares of Common Stock reported on this Schedule 13D) is the beneficial owner of the Common Stock referred to herein for purposes of Section 13(d) of the Securities Exchange Act of 1934, as amended, or for any other purpose, and such beneficial ownership is expressly disclaimed.

(c) Except as set forth herein, none of the Reporting Persons have engaged in any transactions during the past sixty days.

(d) Not applicable.

(e) Not applicable.

Item 6 of the Schedule 13D is hereby supplemented as follows:

In connection with the Proposed 2015 Offering, Blackstone Treasury Holdings III entered into a lock-up agreement, dated April 13, 2015 (the “2015 Lock-Up Agreement”) pursuant to which it agreed that, during the period commencing on the date of the final prospectus relating to the Proposed 2015 Offering and ending 45 days thereafter, it will not, without the prior written consent of the representatives of the underwriters, dispose of or hedge any shares of Common Stock or any securities convertible into or exchangeable for Common Stock, subject to certain exceptions.

The description of the April 2015 Lock-Up Agreement in this Item 6 is not intended to be complete and is qualified in its entirety by reference to such agreement, which is filed as an exhibit hereto and incorporated by reference herein.

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Item 7 of the Schedule 13D is hereby supplemented as follows:

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SIGNATURE

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

Dated: April 15, 2015

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Exhibit 8

FORM OF LOCK-UP LETTER

(BLACKSTONE TREASURY HOLDINGS III L.L.C./BXMT ADVISORS L.L.C.)

April 13, 2015

Citigroup Global Markets Inc.

Merrill Lynch, Pierce, Fenner & Smith

                     Incorporated

J.P. Morgan Securities LLC

Wells Fargo Securities, LLC

c/o Citigroup Global Markets Inc.

388 Greenwich Street

New York, New York 10013

Ladies and Gentleman:

This letter is being delivered to you in connection with the proposed Underwriting Agreement (the “Underwriting Agreement”) among Blackstone Mortgage Trust, Inc., a Maryland corporation (the “Company”), and each of you as representatives of a group of Underwriters named therein (the “Underwriters”), relating to an underwritten public offering (the “Public Offering”) of class A common stock, par value $0.01 per share, of the Company (the “Common Stock”).

To induce the Underwriters to continue their efforts in connection with the Public Offering, the undersigned hereby agrees that, without the prior written consent of each of Citigroup Global Markets Inc., Merrill Lynch, Pierce, Fenner & Smith Incorporated, J.P. Morgan Securities LLC and Wells Fargo Securities, LLC, it will not, during the period commencing on the date hereof and ending 45 days after the date of the final prospectus relating to the Public Offering (the “restricted period”), (1) offer, pledge, sell, contract to sell, sell any option or contract to purchase, purchase any option or contract to sell, grant any option, right or warrant to purchase or otherwise transfer or dispose of, directly or indirectly, any shares of Common Stock or any securities convertible into or exercisable or exchangeable for Common Stock (including, for the avoidance of doubt, any shares held by a controlled affiliate of the undersigned) or (2) enter into any swap or other arrangement that transfers to another, in whole or in part, any of the economic consequences of ownership of the Common Stock, whether any such transaction described in clause (1) or (2) above is to be settled by delivery of Common Stock or such other securities, in cash or otherwise. The foregoing sentence shall not apply (a) to transactions relating to shares of Common Stock or other securities acquired in open market transactions after the closing of the Public Offering, (b) to transfers as a bona fide gift or gifts, including as a result of the operation of law or estate or intestate succession, (c) to transfers to the undersigned’s affiliates or to any investment fund or other entity controlled or managed by the undersigned, (d) to transfers as a distribution to limited partners, members or stockholders of the undersigned, (e) to transfers to any parent, grandparent, stepparent, mother-in-law, father-in-law, spouse, former spouse, sibling, sister-in-law, brother-in-law, son-in-law, daughter-in-law, child, stepchild, grandchild, niece or nephew of the undersigned, including adoptive relationships (each, a “Family Member”) or any dependent of the undersigned, (f) to transfers solely for estate planning purposes to any trust for the direct or indirect benefit of the undersigned or any Family Member or to any corporation, limited liability company, partnership or other entity beneficially owned, directly or indirectly, solely by such trusts, the undersigned or any Family Member, (g) to transfers to a nominee or custodian of a person or entity to whom a disposition or transfer would be permissible under clauses (b) through (f) above or (h) to transfers pursuant to an order of a court or regulatory agency; provided, however, that in the case of any transactions relating to shares of Common Stock or other securities acquired in open market transactions after the closing of the Public Offering pursuant to clause (a) hereunder, no filing under Section 16(a) of the Securities Exchange Act of 1934, as amended (“Section 16”), reporting a reduction in beneficial ownership of shares of Common Stock shall be required or shall be voluntarily made during the restricted period; provided, further, that in the case of any transfer pursuant to clauses

(b) through (g) hereunder (i) the donee, trustee, distributee or transferee, as the case may be, agrees to be bound in writing by the restrictions set forth herein and (ii) no filing under Section 16 reporting a reduction in beneficial ownership of shares of Common Stock shall be required or shall be voluntarily made during the restricted period.

In addition, the undersigned agrees that, without the prior written consent of each of Citigroup Global Markets Inc., Merrill Lynch, Pierce, Fenner & Smith Incorporated, J.P. Morgan Securities LLC and Wells Fargo Securities, LLC, it will not, during the restricted period, make any demand for or exercise any right with respect to the registration of any shares of Common Stock or any security convertible into or exercisable or exchangeable for shares of Common Stock. The undersigned also agrees and consents to the entry of stop transfer instructions with the Company’s transfer agent and registrar against the transfer of the undersigned’s shares of Common Stock unless such transfer is in compliance with the foregoing.

If (1) during the last 17 days of the restricted period the Company issues an earnings release or material news or a material event relating to the Company occurs or (2) prior to the expiration of the restricted period, the Company announces that it will release earnings results during the 16-day period beginning on the last day of the restricted period, the restrictions imposed by this Lock-up Agreement shall continue to apply until the expiration of the 18-day period beginning on the issuance of the earnings release or the occurrence of the material news or material event unless each of Citigroup Global Markets Inc., Merrill Lynch, Pierce, Fenner & Smith Incorporated, J.P. Morgan Securities LLC and Wells Fargo Securities, LLC waive, in writing, such extension. However, such extension will not apply if, within three business days prior to the 15th calendar day before the last day of the applicable 45-day restricted period, the Company delivers a certificate signed by its Chief Executive Officer or Chief Financial Officer, certifying that (i) the Company’s shares of class A common stock are “actively traded securities” as defined in Regulation M, and (ii) the Company meets the requirements set forth in paragraph (a)(1) of Rule 139 under the Securities Act.

The undersigned understands that the Company and the Underwriters are relying upon this Lock-up Agreement in proceeding toward consummation of the Public Offering. The undersigned further understands that this Lock-up Agreement is irrevocable and shall be binding upon the undersigned’s heirs, legal representatives, successors and assigns.

Whether or not the Public Offering actually occurs depends on a number of factors, including market conditions. Any Public Offering will only be made pursuant to an Underwriting Agreement, the terms of which are subject to negotiation between the Company and the Underwriters. This Lock-up Agreement shall automatically terminate on May 15, 2015 if the Public Offering has not been consummated by that date or the Underwriting Agreement (other than the provisions thereof that survive termination) is terminated.

The restrictions described in this Lock-up Agreement shall not apply to the establishment of a trading plan pursuant to Rule 10b5-1 under the Securities Exchange Act of 1934, as amended, provided that no transfers occur under such plan during the restricted period and no public announcement or filing shall be required or voluntarily made by any person in connection therewith.

This Lock-up Agreement shall be governed by and construed in accordance with the laws of the State of New York.

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