John Castle (born 14 January 1940, Croydon, Surrey, England) is an English film, stage, and television actor. He is known for his role as Agrippa Postumus in the BBC television adaptation of I, Claudius (1976) and for playing Geoffrey II, Duke of Brittany, in the film The Lion in Winter (1968). He also played Dr Carrasco as well as the prisoner called "The Duke" in the film Man of La Mancha (1972), and the villainous Paul McDagget in RoboCop 3 (1993).
Born in Croydon, Castle was educated at Brighton College and Trinity College, Dublin, and trained at the Royal Academy of Dramatic Art (RADA). He has been married to Maggie Wadey since 1963. They have one child.