Filed Pursuant to Rule 424(b)(3)

Registration No. 333-164703

KBS REAL ESTATE INVESTMENT TRUST III, INC.

SUPPLEMENT NO. 14 DATED AUGUST 9, 2013

TO THE PROSPECTUS DATED FEBRUARY 6, 2013

This document supplements, and should be read in conjunction with, the prospectus of KBS Real Estate Investment Trust III, Inc. dated February 6, 2013, as supplemented by supplement no. 12 dated July 15, 2013 and supplement no. 13 dated July 15, 2013. As used herein, the terms “we,” “our” and “us” refer to KBS Real Estate Investment Trust III, Inc. and, as required by context, KBS Limited Partnership III, which we refer to as our “Operating Partnership,” and to their subsidiaries. Capitalized terms used in this supplement have the same meanings as set forth in the prospectus. The purpose of this supplement is to disclose information regarding the extension of our primary initial public offering.

Extension of the Primary Initial Public Offering

Our board of directors has extended the closing date of our primary initial public offering to April 24, 2014. We have filed a registration statement on Form S-11 with the SEC to register a follow-on public offering (the “Follow-on Offering”). We can give no assurance that we will commence or complete a Follow-on Offering. Our offering stage will depend on a number of considerations, including the composition of our portfolio and opportunities in the market for real estate investments. We will continue to monitor these factors and we may adjust the termination date of our offering stage as necessary should these factors change.

We plan to continue to offer shares under our dividend reinvestment plan beyond the termination of the offering stage for our primary offering until we have sold 80,000,000 shares through the reinvestment of distributions. In many states, we will need to renew the registration statement or file a new registration statement to continue the dividend reinvestment plan offering for these periods. We may terminate the dividend reinvestment plan offering at any time.

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