Alan III of Rennes (c. 997 – 1 October 1040) (French: Alain III de Bretagne) was Count of Rennes and duke of Brittany, by right of succession from 1008 to his death.

[1] When Richard III, Duke of Normandy died in August 1026, his brother Robert I succeeded him.

[5] Alan apparently took advantage of the resulting turmoil to break free of Norman suzerainty.

[9] In 1037, upon the death of Robert, Archbishop of Rouen, the protection of young William was now left to Alan III and his cousin Gilbert, who tentatively held Normandy together.

[10] On 1 October 1040, while besieging a rebel castle near Vimoutiers in Normandy, Alan III suddenly died.