What was the official document called that Japan signed to formalize its World War II surrender?

The official document was called the Japanese Instrument of Surrender.

Representatives of Japan signed it aboard USS Missouri in Tokyo Bay on 2 September 1945. The document accepted the provisions of the Potsdam Declaration and formally capitulated to the Allied powers, ending the war in the Pacific.

The Japanese Instrument of Surrender was not the same document as the Potsdam Declaration. The declaration, issued in July 1945 by the United States, United Kingdom, and China, demanded Japan's surrender and outlined the consequences of continued resistance. The instrument was the later document that Japan actually signed.

It was also not a peace treaty. The Treaty of San Francisco, signed in 1951, dealt with Japan's postwar peace settlement and came into force in 1952. The surrender instrument ended wartime hostilities, while the later treaty addressed the legal framework of peace and Japan's international status.