What title identified Douglas MacArthur as the Allied occupation's top authority in Japan?

The title identifying Douglas MacArthur as the occupation's top authority was Supreme Commander for the Allied Powers.

MacArthur became Supreme Commander for the Allied Powers, commonly abbreviated SCAP, after Japan's surrender. The title gave him authority to direct the occupation and implement the Allied policies imposed on Japan. He exercised that authority from Tokyo while working through Japanese governmental institutions rather than replacing them with a conventional Allied military government.

MacArthur had already used the title in the surrender ceremony, where he accepted Japan's capitulation on behalf of the Allied nations. His position covered demilitarization, political reform, economic reconstruction, and the prosecution of suspected war criminals. The occupation began in late August 1945 and continued until the Treaty of San Francisco took effect in 1952.

SCAP can also refer to the headquarters and administrative directives associated with MacArthur's command, not only to the individual officeholder. MacArthur was the first and most prominent holder of the title, but the title itself described the Allied command structure.