William I de Cantilupe 1st feudal baron of Eaton (Bray) in Bedfordshire, England, was an Anglo-Norman royal administrator who served as steward of the household to King John and as Baron of the Exchequer.

The feudal barony of Eaton Bray in Bedfordshire was an English feudal barony founded in 1205 when the manor of Eaton was granted by King John to his household steward William I de Cantilupe (d.1239), together with many others, including Aston in Warwickshire. In 1221 Cantilupe built a castle at Eaton, which became the caput of his feudal barony and was described by the monks of nearby Dunstable Priory in the Annals of Dunstable as being "a serious danger to Dunstable and the neighbourhood". The grant was for knight-service of one knight and was in exchange for the manor of Great Coxwell, Berkshire, which had been granted to him previously but the grant was deemed compromised. Eaton had been held at the time of William the Conqueror by the latter's uterine half-brother Odo, Bishop of Bayeux, but later escheated to the crown.