Eudo of Penthièvre (d. 1079) was the brother of Alan III of Brittany (1008-1040). Eudo had been loyal to his brother throughout his reign, but he subsequently spent many years trying to wrest ducal power in Brittany from his nephew and ward, Conan II (1040-66). Eudo was not finally defeated until 1057, but the loyalty of both his eldest son Geoffrey I Boterel and his nephew Geoffrey Grenonat, count of Rennes (1066-84), a natural son of Alan III, remained dubious until their respective deaths, in revolt, in 1093 and 1084.