William Demarest (27 February 1892 – 28 December 1983) was an American character actor.

Born in St. Paul, Minnesota, he was a very prolific film and TV actor, having worked on over 140 films. He worked regularly with director Preston Sturges, becoming part of a "stock" troupe of actors that Sturges repeatedly cast in his films. He started in show business working in vaudeville, then moved on to Broadway. His film career began in 1926 and spanned the decades up to the 1970s. His most famous TV role was in My Three Sons from 1965 to 1972, playing Uncle Charley and replacing William Frawley, whose failing health had made procuring insurance impossible.

Demarest received a single Academy Award nomination, for his supporting role in The Jolson Story, playing Al Jolson's fictional mentor. He had previously shared the screen with the real Al Jolson in The Jazz Singer.