Conan III, also known as Conan of Cornouaille and Conan the Fat (c. 1093-1096 – September 17, 1148), (Breton Konan III a Vreizh, and Konan Kerne) is duke of Brittany, from 1112 to his death.

He is the son of Duke Alan IV and Ermengarde of Anjou.

Conan III allies himself with Stephen of England in Stephen's war against the dispossessed Empress Matilda.