Bill Brennan (June 23, 1893 – June 15, 1924) was an American boxer who fought and lost to World Heavyweight Champion Jack Dempsey in a well attended title fight that ended in a twelfth-round knockout on December 14, 1920, in Madison Square Garden. He lost to Dempsey for the first time in a non-title fight on February 5, 1918, in Milwaukee, Wisconsin, in a sixth-round technical knockout.