Geoffrey I Boterel (died 1093) was a Breton noble and one of the leaders of the Breton Revolt. He was the son of Eudo of Penthièvre, younger brother of Alan III of Brittany. 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 Geoffrey Boterel and his nephew Geoffrey Grenonat, count of Rennes (1066-84), a natural son of Alan III, remained dubious until their respective deaths. Significantly neither Eudo of Penthièvre nor his eldest son Geoffrey I Boterel took part in the Norman conquest of England. When Ralph of Gael fled from England to Brittany in 1076 Geoffrey Grenonat of Rennes was holding the castle of Dol against Duke Hoël, together with Geoffrey I Boterel. Hoël and William the Conqueror jointly invested Dol, but the siege was raised by Philip I of France and Fulk IV of Anjou. Ralph of Gael probably joined the rebels, but the event has been seen as the result of William's support of Archbishop Juhel of Dol, a simoniac whose years of misrule had finally led to his deposition. However, his campaign ended in defeat and the rebellion was quelled.