What was the document Japan signed on September 2, 1945 called?

The document Japan signed on September 2, 1945, was the Japanese Instrument of Surrender.

It was the legal document by which Japan accepted the terms of the Potsdam Declaration and surrendered to the Allied powers. Japanese Foreign Minister Mamoru Shigemitsu signed for the Japanese government, followed by General Yoshijiro Umezu for Imperial General Headquarters.

General Douglas MacArthur signed on behalf of the Allied powers, and representatives of nine Allied nations added their signatures. The ceremony took place aboard USS Missouri in Tokyo Bay. Two versions of the instrument were prepared: one for the Allied governments and one for Japan.

The instrument was not the same thing as a later peace treaty. The Treaty of San Francisco, signed in 1951 and effective in 1952, formally ended the state of war between Japan and most of the Allied countries. The 1945 instrument instead established Japan's immediate military surrender and the framework for Allied occupation.