William E. Gienapp (February 27, 1944 – October 29, 2003) was an American historian noted for his writing on the period of the American Civil War. His prize-winning The Origins of the Republican Party, 1852-1856 (1987) was based on original research and revised the traditional understanding of the political party's origins. Later he wrote Abraham Lincoln and Civil War America: A Biography (2002). He co-authored a widely-used United States history textbook, Nation of Nations, and compiled one of the most widely-used documentary readers on the era of the Civil War, The Civil War and Reconstruction: A Documentary Collection (2001).