The President of the United States at the end of World War II was Harry S. Truman.
You should be clear, however, that Truman was not president for most of the war. He is not the president most connected to WWII. The president most connected to this war is Franklin D. Roosevelt. Roosevelt was president for all of the war until he died in April of 1945. Truman was his vice president, but had only been his vice president for about a very short time. Truman had only been elected vice president in November of 1944 and taken had office in 1945.
Truman, then, was president at the end of the war, but was not in the executive branch for most of the war.