Synetocephalus Fall, 1910 is reviewed and five new species are described: Synetocephalus purshiae Gilbert and Clark, new species; Synetocephalus sagei Gilbert and Clark, new species; Synetocephalus strongi Gilbert and Clark, new species; Synetocephalus variabilis Gilbert and Clark, new species; and Synetocephalus wilcoxi Gilbert and Clark, new species. Synetocephalus is limited to the western United States and the state of Baja California, Mexico. All known species are illustrated, and a key to species is presented.