What was the official document called that formally ended Japan’s participation in World War II?

The official document was called the Japanese Instrument of Surrender.

It was signed on 2 September 1945 aboard USS Missouri in Tokyo Bay. The document formalized Japan’s capitulation and stated that the Japanese government accepted the provisions of the Potsdam Declaration. It also ordered Japanese forces to cease hostilities and surrender to Allied commanders.

Mamoru Shigemitsu signed first for the Japanese government, followed by General Yoshijiro Umezu for Imperial General Headquarters. General Douglas MacArthur signed for the Allied powers, and representatives of eight other Allied nations then added their signatures.

The instrument was not a peace treaty. It was a capitulation document that ended active hostilities and established the framework for Allied control and occupation. A separate peace treaty with Japan, the Treaty of San Francisco, was signed in 1951 and took effect in 1952.