Guillaume de Montfort, (died 27 September 1432,[1] Siena, Italy) was a 15th-century French cleric, Bishop of Saint-Malo and cardinal.
Montfort was born in Dinan, Brittany, the son of, lord of Montfort and Gaël, and Isabeau de la Roche-Bernard, lady of Loudéac (died 1400).
Entering the church at a young age, Montfort was appointed protonotary apostolic and Archdeacon of Dinan.[2] Named Bishop of Saint-Malo on 13 October 1423, the following year he took command of the military forces of Brittany, and advanced on the English besieging Mont Saint-Michel, forcing them to withdraw.
Pope Martin V offered Montfort the bishoprics of Saint-Brieuc in 1424, and of Dol in 1430, but he refused both.
On 29 May 1424, he demonstrated his opposition to John V, Duke of Brittany, who was constructing a dungeon at the château of Saint-Malo, by throwing a stone. Thus he affirmed his rights over the territory, which the duke had appropriated.
He was created a cardinal in pectore by Pope Martin V on 8 November 1430.[2] The pope died the following year; his successor, Pope Eugene IV, announced his nomination in the consistory of 11 March 1432. He received the cardinal's hat and the titular church of cardinal de Sainte-Anastasie on 13 June 1423.[3] [4] He was henceforward known as "the cardinal of Brittany".[5]
While visiting Rome, he travelled secretly to the Council of Basel, against the pope's wishes. He died suddenly en route, possibly poisoned,[2] on 27 September 1432, and was buried in the church of the Cordeliers in Siena.