As filed with the Securities and Exchange Commission on May 9, 2019

Securities Act File No. 33-54126

Investment Company Act File No. 811-07332

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM N-1A

(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

55 East 52^nd Street

New York, New York 10055

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.

Master Investment Portfolio has also executed this Registration Statement.

Explanatory Note

This Post-Effective Amendment No. 295 under the Securities Act of 1933, as amended (the “Securities Act”) (Amendment No. 299 under the Investment Company Act of 1940, as amended (the “1940 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 June 7, 2019, the effectiveness of the registration statement for iShares S&P 500 Index Fund (the “Fund”), filed in Post-Effective Amendment No. 288 on March 11, 2019, pursuant to paragraph (a) of Rule 485 under the Securities Act.

This Post-Effective Amendment No. 295 incorporates by reference the information contained in Parts A, B and C of Post-Effective Amendment No. 288 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 May 9, 2019.

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.

SIGNATURES

Master Investment Portfolio has duly caused this Post-Effective Amendment to the Registration Statement of BlackRock Funds III 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 May 9, 2019.

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