Hoel III of Cornwall (died 1156) was duke of Brittany, from 1148 to his death. He was son of Duke Conan III and Maud of England, an illegitimate daughter of King Henry I of England. He was disinherited by his father, who claimed that Hoel was a bastard and not his legitimate son at all. Thusly his sister Bertha became heiress to Duke Conan's lands in Brittany, alongside her husband, while Hoel was allowed to remain Count of Nantes. He was accused by St. Bernard of Clairvaux of having an incestuous affair with his (half?)sister Bertha.

In 1156 the Nantaise rebelled against Hoel and drove him out of the country; he died shortly afterward, possibly at the Cistercian abbey of Melleray .

See also: Dukes of Brittany family tree