Robert Fitzwalter (died 9 December 1235) was the leader of the baronial opposition against King John, and one of the twenty-five sureties of Magna Carta. He was feudal baron of Little Dunmow, Essex and constable of Baynard's Castle, in London, to which was annexed the hereditary office of castellain and chief banneret of the City of London. Part of the official aristocracy created by Henry I and Henry II, he served John in the wars in Normandy, in which he was taken prisoner by King Philip II of France and forced to pay a heavy ransom.