Which U.S. president announced September 2 as the official V-J Day in 1945?

Harry S. Truman announced September 2 as the official V-J Day in 1945.

Truman made the announcement after Japan's formal surrender ceremony aboard USS Missouri in Tokyo Bay. September 2 marked the signing of the Instrument of Surrender, while August 15 marked Emperor Hirohito's broadcast announcing Japan's acceptance of the Allied terms.

This distinction explains why V-J Day has more than one associated date. In the United States, the formal signing date became the official commemoration. In several other countries, especially in the British Commonwealth, August 15 is more commonly observed because it marked the public announcement of Japan's surrender.

Truman had become president in April 1945 after Franklin D. Roosevelt died. He authorized the use of atomic bombs against Hiroshima and Nagasaki and led the United States through the final months of the war and the surrender period.