Alan III (c. 997 – 1 October 1040) was count of Rennes and duke of Brittany from 1008 to his death.
Alan was the son of Geoffrey I of Brittany and Hawise of Normandy. He succeeded his father in 1008. Because he was still a minor at his father's death, his mother acted as regent of Brittany while her brother Richard II, Duke of Normandy assumed guardianship over Brittany. In 1018 Alan married Bertha of Blois. They had a son, Conan II, and a daughter, Hawise. Alan apparently took advantage of the resulting turmoil to break free of Norman suzerainty. Facing an invasion from Normandy via land and from Duke Robert's fleet, Archbishop Robert of Rouen (uncle of Hawise and Richard) mediated a truce between his two great-nephews at Mont Saint-Michel where Alan swore fealty to his cousin Robert. When he left Normandy for the Holy Land, Robert appointed Alan to be a guardian of his young son William.
Alan III also assisted Count Herbert I of Maine in his wars with
Avesgaud, bishop of Le Mans, and was with the count in his attack on Avesgaud's castle at La Ferté-Bernard destroying the castle and causing Avesgaud to flee.
In 1037, upon the death of Archbishop Robert, the protection of William was left to Alan and Gilbert, who tentatively held Normandy together. They appointed Mauger to the now vacant see of Rouen and his brother William as count of Arques, attempting to gain their support for Duke William.
On 1 October 1040, while besieging a rebel castle near Vimoutiers in Normandy, Alan III suddenly died.