As filed with the Securities and Exchange Commission on August 4, 2023

1933 Act Registration No. 33-54126

1940 Act Registration No. 811-07332

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

REGISTRATION STATEMENT

UNDER

and/or

REGISTRATION STATEMENT

UNDER

(Check appropriate box or boxes)

BlackRock Funds III

(Exact Name of Registrant as Specified in Charter)

400 Howard Street

San Francisco, California 94105

(Address of Principal Executive Office)

Registrant’s Telephone Number, including Area Code: (800) 441-7762

John M. Perlowski

BlackRock Funds III

50 Hudson Yards

New York, New York 10001

United States of America

(Name and Address of Agent for Service)

Copies to:

It is proposed that this filing will become effective (check appropriate box)

If appropriate, check the following box:

Title of Securities Being Registered: Shares of Beneficial Interest.

Explanatory Note

This Post-Effective Amendment No. 359 under the Securities Act of 1933, as amended (the “Securities Act”) to the registration statement on Form N-1A (the “Registration Statement”) of BlackRock Funds III (the “Registrant”) is being filed pursuant to paragraph (b)(1)(iii) of Rule 485 under the Securities Act solely for the purpose of delaying, until September  6, 2023, the effectiveness of the registration statement of BlackRock Diversified Fixed Income Fund, filed in Post-Effective Amendment No. 358 on June 8, 2023, pursuant to paragraph (a) of Rule 485 under the Securities Act.

This Post-Effective Amendment No. 359 incorporates by reference the information contained in Parts A, B and C of Post-Effective Amendment No. 358 to the Registrant’s Registration Statement.

SIGNATURES

Pursuant to the requirements of the Securities Act of 1933 and the Investment Company Act of 1940, the Registrant certifies that it meets all the requirements for the effectiveness of this Post-Effective Amendment to its Registration Statement pursuant to Rule 485(b)(1)(iii) under the Securities Act of 1933 and has duly caused this Post-Effective Amendment to its Registration Statement to be signed on its behalf by the undersigned, thereunto duly authorized, in the City of New York and the State of New York, on August 4, 2023.

Pursuant to the requirements of the Securities Act of 1933, this Post-Effective Amendment to its Registration Statement has been signed below by the following persons in the capacities and on the dates indicated.