What was the formal written agreement that documented Japan's surrender in World War II called?

The formal written agreement documenting Japan’s surrender was the Japanese Instrument of Surrender.

The document was signed on 2 September 1945 aboard USS Missouri in Tokyo Bay. It stated that Japan accepted the provisions of the Potsdam Declaration and surrendered unconditionally to the Allied powers. The signing formally ended hostilities between Japan and the Allies.

Japanese Foreign Minister Mamoru Shigemitsu signed for the Japanese government, and General Yoshijiro Umezu signed for Imperial General Headquarters. General Douglas MacArthur signed for the Allied powers, followed by national representatives from eight other Allied governments.

The Instrument of Surrender was not the same as the Treaty of San Francisco. The surrender instrument ended the war and established the immediate capitulation, while the 1951 peace treaty formally ended the state of war between Japan and many Allied nations and set postwar arrangements. Copies of the surrender document were distributed and preserved in national archives.