William de St-Calais (also Calais or Carileph or Carilef; died 1096) is a medieval Norman monk, abbot of the abbey of Saint-Vincent in Le Mans in Maine, who is nominated by King William I of England as Bishop of Durham in 1080.

During his term as bishop, St-Calais replaces the canons of his cathedral chapter with monks, and begins the construction of Durham Cathedral.

In addition to his ecclesiastical duties, he serves as a commissioner for the Domesday Book.

He is 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.

However, when the king's uncle, Odo of Bayeux, raises a rebellion against the king in 1088, St-Calais is implicated in the revolt.

William Rufus lays siege to St-Calais in the bishop's stronghold of Durham, and later puts 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 is allowed to go into exile after his castle at Durham is surrendered to the king.

He goes to Normandy, where he becomes a leading advisor to Robert Curthose, Duke of Normandy, the elder brother of William Rufus.

By 1091, St-Calais has returned to England and regained royal favor.

In England, St-Calais once more becomes a leading advisor to the king.

In 1093, he negotiates with Anselm, Abbot of Bec, concerning Anselm's becoming Archbishop of Canterbury; in 1095 it is St-Calais who prosecutes the royal case against Anselm after he has become archbishop.

During his bishopric, St-Calais stocks the cathedral library with books, especially canon law texts.

He is also active in defending the north of England against Scots raids.

Before his death, he had made his peace with Anselm, who blesses and consoles St-Calais on his deathbed.