Bertha of Cornouaille ( fl. 1125-55), also known as Bertha of Brittany (Breton: Berthe Breizh), was hereditary Duchess of Brittany between 1148 until her death and Dowager Countess of Richmond. Bertha was the eldest daughter of Conan III of Brittany by Maude, the illegitimate daughter of King Henry I of England. She was the last member of the Breton House of Cornouaille to reign over Brittany.
Bertha was the daughter of Duke Conan III of Brittany. She married the son of Stephen of Treguier, Alan the Black and she lived in England with Alan until his death in 1146. Alan would eventually become Earl of Richmond. After Alan's death she returned to Brittany as Dowager Countess of Richmond and eventually married Odo, Viscount of Porhoet. When her father Duke Conan III died, on his deathbed Conan III renounced Bertha's brother Hoèl as heir, and designated Bertha as his heiress. On Conan III's death she became hereditary Duchess of Brittany.
In her first marriage, by 1138, Bertha was married to Alan le Noir Bertha and Alan had three children:
- Conan IV, Duke of Brittany, b. 1138, their son and heir, as Duke of Brittany and Earl of Richmond
- Constance, who married Alan III, Vicomte of Rohan
- Enougen, abbess of St. Sulpice
Bertha married her second husband, Odo, Viscount of Porhoet in about 1148. Bertha and Odo had three children:
- Adelaide (died in 1220), Abbess of Fontevrault
- Alix, mistress of Henry II, King of England
- Cokayne, G. E., edited by H. A. Doubleday, Geoffrey H. White, & Lord Howard de Walden, The Complete Peerage, London, 1945, vol. x, p. 788.
- ^ Cawley, Charles (12 June 2011), Brittany, Dukes & Nobility: Eudes (II) de Porhoët, Medieval Lands database, Foundation for Medieval Genealogy,
- Cawley, Charles (12 June 2011), Brittany, Dukes & Nobility: Alix de Porhoët, Medieval Lands database, Foundation for Medieval Genealogy,
Bertha died between 1158 and 1164, and with her death the ducal throne passed to her son Conan.