Mitsusmasu Yonai (1880-1948) was an Admiral in the Imperial Japanese Navy, with experience both as a commander at sea and a Russian intelligence specialist. He became Navy Minister in 1937-1939, then Prime Minister of Japan between January and July 1940, opposing war with the United States. He rejoined the Cabinet, as Navy Minister, between July 1944 and December 1945, as the Navy was crushed.
As a member of the inner cabinet and peace faction, he supported unconditional surrender at the 9 August 1945 conference. A discussion between Yonai and Deputy Chief of Staff Sokichi Takagi, two days after Hiroshima, shows him as more concerned with the threat of a domestic uprising than the impact of the nuclear attack. Yonai mentioned Hiroshima in the context of aggravating the domestic situation, not as a primary event. 
Later, he signed the surrender document on behalf of the Navy Ministry.
- Robert Joseph Charles Butow (1954), Japan's decision to surrender, Stanford University Press, p. 38
- Ward Wilson (Spring 2007), "The Winning Weapon? Rethinking Nuclear Weapons in Light of Hiroshima", International Security 31 (4): 162–179