The 109th United States Congress was a meeting of the legislative branch of the United States federal government, composed of the United States Senate and the United States House of Representatives, from January 3, 2005, to January 3, 2007, during the fifth and sixth years of George W. Bush's presidency. House members were elected in the 2004 elections on November 2, 2004. Senators were elected in three classes in the 2000 elections on November 7, 2000, 2002 elections on November 5, 2002, or 2004 elections on November 2, 2004. The apportionment of seats in the House of Representatives was based on the 2000 United States census.

The 2004 United States House of Representatives elections were held on November 2, 2004, to elect all 435 seats of the chamber. It coincided with the re-election of President George W. Bush as well as many Senate elections and gubernatorial elections. Prior to the election in the 108th Congress, Republicans held 227 seats, Democrats held 205, with two Republican vacancies and one independent. As a result of this election, the 109th Congress began composed of 232 Republicans, 201 Democrats, one independent, and one vacancy.