Conan III, also known as Conan of Cornouaille and Conan the Fat (Breton: Konan III a Vreizh, and Konan Kerne; c. 1093–1096 – September 17, 1148) was duke of Brittany, from 1112 to his death. He was the son of Duke Alan IV and Ermengarde of Anjou.
On his death-bed in 1148, Conan III disinherited Hoel from succession to the Duchy, stating that he was illegitimate and no son of his. By this surprise move Bertha became his heiress and successor. However, Hoel was to retain the county of Nantes.