Conan II of Rennes (c. 1033 – 11 December 1066) was Duke of Brittany, from 1040 to his death. Conan was the eldest child and heir of Alan III, Duke of Brittany by his wife Bertha of Blois, and member of the House of Rennes. He was the elder brother of Hawise, who succeeded him as suo jure duchess.

In 1065, before his invasion of Anglo-Saxon England, William of Normandy sent word to the surrounding counties, including Brittany, warning them against attacking his lands, on the grounds that his mission bore the papal banner. Conan promptly informed the duke that he would definitely take the opportunity to invade the latter's duchy. In the history of conflicts between Brittany and Anjou, Pouancé had served as the "Breton March" or border town. During Conan's 1066 campaign against Anjou, he took Pouancé and Segré, and arrived in Château-Gontier, where he was found dead after donning poisoned riding gloves. Duke William was widely suspected.[2] He was also asked to help William the Conqueror on his 1066 conquest of England, but refused, saying that the Normans poisoned his father in 1040.

Conan II died leaving no known issue. It is possible he died because the gloves he was wearing were poisoned, and he swallowed the poison when he wiped his mouth with the glove

The Battle of Dinan occurred in 1065. Harold fought on the side of Duke William, whose army had chased Duke Conan from Dol-de-Bretagne to Rennes. Duke Conan finally surrendered at Château de Dinan, Brittany.
The battle is recalled in the Bayeux Tapestry.