UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D

Under the Securities Exchange Act of 1934

(Amendment No. 6)*

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)

January 1, 2014

(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. 6 (“Amendment No. 6”) 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 (“Blackstone Mortgage Trust”), 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 and Amendment No. 5 to Schedule 13D filed on October 11, 2013 (collectively, the “Schedule 13D”). Capitalized terms used but not defined in this Amendment No. 6 shall have the same meanings ascribed to them in the Schedule 13D.

Item 2. Identity and Background

Item 2 of the Schedule 13D is hereby amended and restated as follows:

(a) – (b) This Schedule 13D is being filed by:

(c) The principal business of Blackstone Treasury Holdings III is performing certain treasury related activities for Blackstone Holdings III and other affiliated Blackstone entities. Blackstone Holdings III is performing the functions of, and serving as, the sole member of Blackstone Treasury Holdings III and other affiliated Blackstone entities. The principal business of Blackstone Holdings III GP L.P. is performing the functions of, and serving as, the general partner of Blackstone Holdings III and other affiliated Blackstone entities. The principal business of Blackstone Holdings III GP Management L.L.C. is performing the functions of, and serving as, the general partner of Blackstone Holdings III GP L.P. and other affiliated Blackstone entities. The principal business of Blackstone is performing the functions of, and serving as, the managing member of Blackstone Holdings III GP Management L.L.C. and other affiliated Blackstone entities. The principal business of Blackstone Group Management L.L.C. is performing the functions of, and serving as, the general partner of Blackstone.

The principal occupation of Mr. Schwarzman is serving as an executive of Blackstone Group Management L.L.C.

(d) During the last five years, none of the Reporting Persons has been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors).

(e) During the last five years, none of the Reporting Persons has been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction and, as a result of such

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proceeding, was or is subject to a judgment, decree or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

(f) See Item 2(a)-(b) above for citizenship or place of organization, as applicable, of each of the Reporting Persons.

Item 4. Purpose of Transaction.

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

On January 1, 2014, Blackstone Holdings III transferred 2,460,784 shares of Common Stock to its wholly owned subsidiary, Blackstone Treasury Holdings III, for financial reporting purposes.

Item 5. Interest in Securities of 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 339,431 shares of Common Stock and Mr. Schwarzman directly holds 22,386 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, 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 29,501,651 shares of Common Stock outstanding, as of January 10, 2014, based on information provided by Blackstone Mortgage Trust, and the issuance of 9,775,000 additional shares of Common Stock pursuant to an underwritten public offering by Blackstone Mortgage Trust expected to close on January 14, 2014. Blackstone Treasury Holdings III directly holds approximately 6.3% of the shares of Common Stock outstanding (or approximately 8.3% if the additional shares of Common Stock are not issued), the Manager directly holds approximately 0.9% of the shares of Common Stock

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outstanding (or approximately 1.2% if the additional shares of Common Stock are not issued) and Mr. Schwarzman directly holds approximately 0.06% of the shares of Common Stock outstanding (or approximately 0.08% if the additional shares of Common Stock are not issued).

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 of 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. Contracts, Arrangements, Understandings or Relationships With Respect to Securities of the Issuer.

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

In connection with the above mentioned transfer, Blackstone Treasury Holdings III has entered into a lock-up letter, dated January 1, 2014, which is filed as an exhibit hereto and incorporated by reference herein, pursuant to which it agreed that, until January 18, 2014, it will not dispose of or hedge the Common Stock. Blackstone Holdings III had entered into a similar lock-up letter with the representatives of the underwriters in connection with the underwritten public offering on November 19, 2013 of Blackstone Mortgage Trust’s convertible senior notes, pursuant to which Blackstone Holdings III is permitted, among other things, to transfer the Common Stock to its affiliates or to any investment fund or other entity controlled or managed by it.

Item 7. Material to be Filed as Exhibits.

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

11. Blackstone Mortgage Trust, Inc. Lock-Up Letter dated as of November 12, 2013 of Blackstone Holdings III L.P.

12. Joint Filing Agreement dated January 10, 2014 among the Reporting Persons (filed herewith).

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13. Blackstone Mortgage Trust, Inc. Lock-Up Letter dated as of January 1, 2014 of Blackstone Treasury Holdings III L.L.C.

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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: January 10, 2014

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Exhibit 11

LOCK-UP LETTER

BLACKSTONE HOLDINGS III L.P.

November 12, 2013

Citigroup Global Markets Inc.

Merrill Lynch, Pierce, Fenner & Smith

                    Incorporated

J.P. Morgan 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 convertible senior notes.

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 and J.P. Morgan Securities LLC, it will not, during the period commencing on the date hereof and ending 60 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 class A common stock, par value $0.01 per share, of the Company (the “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 to (a) transactions relating to shares of Common Stock or other securities acquired in open market transactions after the closing of the Public Offering, (b) transfers as a bona fide gift or gifts, including as a result of the operation of law or estate or intestate succession, (c) transfers to the undersigned’s affiliates or to any investment fund or other entity controlled or managed by the undersigned, (d) as a distribution to limited partners, members or stockholders of the undersigned, (e) 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) 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 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) 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 and J.P. Morgan 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 and J.P. Morgan 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 60-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 November 30, 2013 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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Exhibit 12

JOINT FILING AGREEMENT

The undersigned hereby agree that they are filing the statement on Schedule 13D to which this agreement is included as an exhibit jointly pursuant to Rule 13d-1(k)(1). Each of them is responsible for the timely filing of such statement and any amendments thereto, and for the completeness and accuracy of the information concerning such person contained therein; but none of them is responsible for the completeness or accuracy of the information concerning the other persons making the filing, unless such person knows or has reason to believe that such information is inaccurate.

In accordance with Rule 13d-1(k)(1) promulgated under the Securities and Exchange Act of 1934, as amended, the undersigned hereby agree to the joint filing with each other on behalf of each of them to such statement with respect to the common stock of Blackstone Mortgage Trust, Inc. beneficially owned by each of them. Each of the undersigned hereby expressly authorizes each other party to file on its behalf any and all amendments to such statement. This Joint Filing Agreement shall be included as an exhibit to such statement.

Dated: January 10, 2014

[Blackstone Joint Filing Agreement]

[Blackstone Joint Filing Agreement]

Exhibit 13

LOCK-UP LETTER

BLACKSTONE TREASURY HOLDINGS III L.L.C.

January 1, 2014

Citigroup Global Markets Inc.

Merrill Lynch, Pierce, Fenner & Smith

                    Incorporated

J.P. Morgan Securities LLC

c/o Citigroup Global Markets Inc.

388 Greenwich Street

New York, New York 10013

Ladies and Gentlemen:

Reference is made to the Lock-Up Letter (the “Lock-Up Letter”) delivered to you by Blackstone Holdings III L.P. (“Holdings III”), dated as of November 12, 2013, pursuant to the Underwriting Agreement (the “Underwriting Agreement”), dated as of November 19, 2013, 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 the underwritten public offering of convertible senior notes. In accordance with the terms of the Lock-Up Letter, the undersigned, an affiliate of Holdings III, agrees to the restrictions below in respect of 2,460,784 shares of Common Stock (as defined below) transferred to the undersigned by Holdings III pursuant to clause (c) of the second paragraph of the Lock-Up Letter.

The undersigned hereby agrees that, without the prior written consent of each of Citigroup Global Markets Inc., Merrill Lynch, Pierce, Fenner & Smith Incorporated and J.P. Morgan Securities LLC, it will not, during the period commencing on the date hereof and ending January 18, 2014 (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 class A common stock, par value $0.01 per share, of the Company (the “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 to (a) transactions relating to shares of Common Stock or other securities acquired in open market transactions after November 25, 2013, (b) transfers as a bona fide gift or gifts, including as a result of the operation of law or estate or intestate succession, (c) transfers to the undersigned’s affiliates or to any investment fund or other entity controlled or managed by the undersigned, (d) as a distribution to limited partners, members or stockholders of the undersigned, (e) 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) 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 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) 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 November 25, 2013, 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 and J.P. Morgan 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 and J.P. Morgan 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 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 this Lock-up Agreement is irrevocable and shall be binding upon the undersigned’s heirs, legal representatives, successors and assigns.

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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