Clarence William Nelson II is an American politician and attorney serving as the administrator of the National Aeronautics and Space Administration (NASA). Nelson previously served as a United States senator from Florida from 2001 to 2019. A member of the Democratic Party, he previously served in the Florida House of Representatives from 1972 to 1978 and in the United States House of Representatives from 1979 to 1991. In January 1986, Nelson became the second sitting member of U.S. Congress to fly in space, after Senator Jake Garn, when he served as a payload specialist on mission STS-61-C aboard the Space Shuttle Columbia. Before entering politics he served in the U.S. Army Reserve during the Vietnam War.

The administrator of the National Aeronautics and Space Administration is the highest-ranking official of NASA, the national space agency of the United States. The administrator is NASA's chief decision maker, responsible for providing clarity to the agency's vision and serving as a source of internal leadership within NASA. The office holder also has an important place within United States space policy, and is assisted by a deputy administrator.