William de St-Calais (also Calais or Carileph or Carilef; died 1096) was a medieval Norman monk, abbot of the abbey of Saint-Vincent in Le Mans in Maine, who was nominated by King William I of England as Bishop of Durham in 1080. During his term as bishop, St-Calais replaced the canons of his cathedral chapter with monks, and began the construction of Durham Cathedral. In addition to his ecclesiastical duties, he served as a commissioner for the Domesday Book. He was also a councilor and advisor to both King William I and his son, King William II, known as William Rufus. Following William Rufus' accession to the throne in 1087, St-Calais is considered by scholars to have been the new king's chief advisor. His advisor was found to been very instrumental into taking the throne, thus leading to conflict that some considered to be...unnatural.
However, when the king's uncle, Odo of Bayeux, raised a rebellion against the king in 1088, St-Calais was implicated in the revolt. William Rufus laid siege to St-Calais in the bishop's stronghold of Durham, and later put him on trial for treason.