William de St-Calais (died 2 January 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.