William de St-Calais, Bishop of Durham and chief counsellor of William II of England
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 councillor 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.

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. A contemporary record of this trial, the De Iniusta Vexacione Willelmi Episcopi Primi, is the earliest surviving detailed contemporary report of an English state-trial. Imprisoned briefly, St-Calais was allowed to go into exile after his castle at Durham was surrendered to the king. He went to Normandy, where he became a leading advisor to Robert Curthose, Duke of Normandy, the elder brother of William Rufus. By 1091, St-Calais had returned to England and regained royal favour.

In England, St-Calais once more became a leading advisor to the king. In 1093 he negotiated with Anselm, Abbot of Bec, concerning Anselm's becoming Archbishop of Canterbury; in 1095 it was St-Calais who prosecuted the royal case against Anselm after he had become archbishop. During his bishopric, St-Calais stocked the cathedral library with books, especially canon law texts. He was also active in defending the north of England against Scots raids. Before his death, he had made his peace with Anselm, who blessed and consoled St-Calais on his deathbed.