Bertha of Cornouaille (fl. 1125–56), also known as Bertha of Brittany (Breton: Berthe Breizh), was the Duchess of Brittany between 1148 until her death and Dowager Countess of Richmond. Bertha was the elder daughter of Conan III of Brittany by Maude, the illegitimate daughter of King Henry I of England.^[1] 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,^[2] and she lived in England with Alan until his death in 1146. Alan would eventually become Earl of Richmond. On his deathbed, Conan III renounced Bertha's brother Hoel as heir^[2] and designated Bertha as his heiress, Duchess of Brittany.^[3]

In her first marriage, by 1138, Bertha was married to Alan the Black.^{[lower-alpha 1]}

Bertha and Alan had three children:

• Conan IV, b. 1138, their son and heir, as Duke of Brittany and Earl of Richmond^[3]
• Constance, who married Alan III, Viscount of Rohan^[5]
• Enoguen, abbess of St. Sulpice

Bertha married her second husband, Odo, Viscount of Porhoet, in about 1148.^[2][6] Bertha and Odo had three children:

• Geoffroy
• Adelaide (died in 1220), Abbess of Fontevrault, mistress of Henry II, King of England^[7][5]
• Alix^{[lower-alpha 2]}

Bertha died between 1158 and 1164, and with her death the ducal throne passed to her son Conan.^{[lower-alpha 3]}