The 1866 United States general election occurred in the middle of National Union President Andrew Johnson's term, during the Third Party System and Reconstruction. Members of the 40th United States Congress were chosen in this election. Arkansas, Florida, Alabama, North Carolina, Louisiana, and South Carolina were re-admitted during the 40th Congress. The Republican Party maintained a dominant majority in both houses of Congress.
In the House, both parties picked up several seats, but Republicans retained a majority. Republican Schuyler Colfax won election as Speaker of the House.
In the Senate, Republicans won massive gains and increased their already-dominant majority, while Democrats suffered slight losses.