What was the official name of the document Japan signed to end the war?

The official document Japan signed to end the war was the Japanese Instrument of Surrender.

The instrument was signed aboard USS Missouri in Tokyo Bay on September 2, 1945. It formally recorded Japan’s acceptance of the Potsdam Declaration and ordered Japanese forces to cease hostilities and surrender. It also required Japanese forces to obey the instructions of the Supreme Commander for the Allied Powers.

Mamoru Shigemitsu signed for the Japanese government, and General Yoshijirō Umezu signed for the Imperial General Headquarters. Douglas MacArthur signed for the Allied powers, followed by representatives of the United States and other Allied nations.

The document is called an instrument rather than a peace treaty because it recorded capitulation and military surrender. A full peace treaty came later: the Treaty of San Francisco was signed in 1951 and entered into force in 1952. Therefore, the 1945 instrument ended active Japanese resistance, while the later treaty established a broader legal postwar settlement.