The House of Montfort was a French noble family, which reigned in the Duchy of Brittany from 1365 to 1514. It was a cadet branch of the Breton House of Dreux, itself a branch of the House of Dreux; it was thus ultimately part of the Capetian dynasty. It should not be confused with the older House of Montfort which ruled as Counts of Montfort-l'Amaury. It succeeded the Brittany branch of the House of Dreux, invoking already in 1341 a right to succeed John III, Duke of Brittany. A war ensued, ultimately won by Montforts in 1364.