Bertrade de Montfort (died 1117) was the daughter of Simon de Montfort-l'Amaury and Agnes, Countess of Evreux. Her brother was Amaury III de Montfort .

In 1089 she married Count Fulk IV of Anjou, and they became the parents of a son, but Bertrade left Fulk and took up with King Philippe. Philippe married her on May 15, 1092, despite the fact that they both had spouses living. He was so enamoured of Bertrade that he refused to leave her even when threatened with excommunication. Pope Urban II did excommunicate him in 1095, and Philip was prevented from taking part in the First Crusade. Astonishingly, Bertrade persuaded Philippe and Fulk to be friends.

Bertrade and Philippe had four children together:

1. Philippe de France, Count of Mantes (c. 1093 [?]-1123)
2. Fleury de France, Lord of Nangis (1095 [?]-1118)
3. Cécile (1097-1145), married (1) Tancred, Prince of Galilee; married (2) Pons of Tripoli
4. Eustachie (c. 1095-?), married Jean, Count of Etampes

According to Orderic Vitalis, Bertrade was anxious that one of her sons succeed Philippe, and sent a letter to King Henry I of England asking him to arrest her stepson Louis. Orderic also claims she sought to kill Louis first through the arts of sorcery, and then through poison. Whatever the truth of these allegations, Louis succeeded Philippe in 1108. Bertrade lived on until 1117; William of Malmesbury says: "Bertrade, still young and beautiful, took the veil in the abbey of Fontrevault , always charming to men, pleasing to God, and like an angel.". Her son from her first marriage was Fulk V of Anjou.

Sources

• Orderic Vitalis
• William of Malmesbury