William Nelson is an American politician of the Democratic Party serving as the senior United States Senator from Florida, in office since 2001.

Nelson began his career in the Florida House of Representatives, where he served from 1972 to 1978. He then served in the United States House of Representatives from 1979 to 1991.

In January 1986, Nelson became the second sitting member of the United States Congress to fly in space. He flew as a Payload Specialist on the Space Shuttle Columbia.