What was the Allied document Japan accepted before signing its formal surrender in 1945?

Japan accepted the Potsdam Declaration before signing its formal surrender in 1945.

Issued on July 26, 1945, the declaration demanded Japan's unconditional surrender of its armed forces and outlined broad conditions for the country's future. It was issued by the United States, the United Kingdom, and China during the final months of World War II; the Soviet Union joined the war against Japan later.

The declaration warned Japan of “prompt and utter destruction” if it refused. Japan initially did not accept the terms, but Emperor Hirohito eventually directed the government to agree after the atomic bombings and the Soviet declaration of war.

The Japanese Instrument of Surrender specifically recorded Japan's acceptance of the Potsdam Declaration. The declaration therefore formed the political basis for the surrender, while the later instrument provided the formal legal act of capitulation.