Was Harry Truman was president of the United States when the atomic bomb was dropped on Hiroshima?

Yes, he was the president, because President Roosevelt died, and Harry Truman took his place in result of his death.

President Harry S. Truman, warned by some of his advisers that any attempt to invade Japan would result in horrific American casualties, ordered that the new weapon be used to bring the war to a speedy end. On August 6, 1945, the American bomber Enola Gay dropped a five-ton bomb over the Japanese city of Hiroshima