BLACKROCK FUNDS III

iShares U.S. Aggregate Bond Index Fund

(the “Fund”)

Supplement dated August 1, 2025 to the Summary Prospectuses and the

Prospectuses of the Fund, each dated April 30, 2025, as amended or supplemented to date

Effective immediately, the following changes are made to the Fund’s Summary Prospectuses and Prospectuses, as applicable:

The section of the Summary Prospectuses entitled “Key Facts About iShares U.S. Aggregate Bond Index Fund — Portfolio Managers” and the section of the Prospectuses entitled “Fund Overview — Key Facts About iShares U.S. Aggregate Bond Index Fund — Portfolio Managers” are deleted in their entirety and replaced with the following:

 

Name		Portfolio Manager of the Master Portfolio Since		Title
James Mauro		2011		Managing Director of BlackRock, Inc.
Jonathan Graves		2025		Managing Director of BlackRock, Inc.
Marcus Tom		2025		Director of BlackRock, Inc.

The section of the Prospectuses entitled “Details About the Fund — Details About the Fund’s Investment Strategies — About the Portfolio Management Team of the Fund/Master Portfolio” is deleted in its entirety and replaced with the following:

 

ABOUT THE PORTFOLIO MANAGEMENT TEAM OF THE FUND/MASTER PORTFOLIO

The Master Portfolio is managed by a team of financial professionals. James Mauro, Jonathan Graves and Marcus Tom are the portfolio managers and are jointly and primarily responsible for the day-to-day management of the Master Portfolio. Please see “Management of the Fund — Portfolio Managers” for additional information about the portfolio management team.

The section of the Prospectuses entitled “Management of the Fund — Portfolio Managers” is deleted in its entirety and replaced with the following:

Information regarding the portfolio managers of the Master Portfolio is set forth below. Further information regarding the portfolio managers, including other accounts managed, compensation, ownership of Fund shares, and possible conflicts of interest, is available in the Fund’s SAI.

 

Portfolio Manager		Primary Role		Since		Title and Recent Biography
James Mauro		Jointly and primarily responsible for the day-to-day management of the Master Portfolio’s portfolio, including setting the Master Portfolio’s overall investment strategy and overseeing the management of the Master Portfolio.		2011		Managing Director of BlackRock, Inc. since 2015 and Global Head of Index Fixed Income Portfolio Management at BlackRock, Inc. since 2025.
Jonathan Graves		Jointly and primarily responsible for the day-to-day management of the Master Portfolio’s portfolio, including setting the Master Portfolio’s overall investment strategy and overseeing the management of the Master Portfolio.		2025		Managing Director of BlackRock, Inc. since 2013 and Head of Credit within the Index Fixed Income Portfolio Management team in the Americas at BlackRock, Inc. since 2014.

Portfolio Manager		Primary Role		Since		Title and Recent Biography
Marcus Tom		Jointly and primarily responsible for the day-to-day management of the Master Portfolio’s portfolio, including setting the Master Portfolio’s overall investment strategy and overseeing the management of the Master Portfolio.		2025		Director of BlackRock, Inc. since 2019 and Head of Index Fixed Income Portfolio Management team in Atlanta at BlackRock since 2024.

Shareholders should retain this Supplement for future reference.

PR2-USAGG-0825SUP

 

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