Michael Newbold "Mike" Castle (born July 2, 1939) is an American politician who was Governor of Delaware (1985-92) and the U.S. Representative for Delaware's at-large congressional district (1993-2011). He is a member of the Republican Party.

The district includes the entire state of Delaware and is the oldest intact surviving district in the nation. He was the longest-serving U.S. Representative in the state's history. Prior to his election to Congress, Castle served as a member of the Delaware General Assembly, starting in the State House of Representatives (1966–1967) and then followed by election to the State Senate (1968–1976). He was the 20th Lieutenant Governor of Delaware from 1981 to 1985, and the 69th Governor of Delaware from 1985 to 1992.

On October 6, 2009, Castle announced his candidacy in the 2010 special election for the seat in the United States Senate held by Democrat Ted Kaufman. Kaufman, appointed by Governor Ruth Ann Minner to fill the vacancy created by Joe Biden (who resigned to become Vice President of the United States), was not a candidate in the special election. The election would determine who would fill the balance of Biden's term, which would end on January 3, 2015. In one of the most surprising election results of the 2010 campaign season, Castle was defeated in the Republican primary for the US Senate seat by Christine O'Donnell. He would have been heavily favored in the general election against Democrat Chris Coons, who went on to easily beat O'Donnell by 17%.