Cynhaiarn was a 5th century Pre-Congregational Saint of Wales, and the brother of Aelhaiarn.
Very little is known of his life ather than he was a prince of the Powysian dynasty descended from Vortigern, king of Britain, and He was a brother of Llwchaiarn and Aelhaiarn.
He was a cousin and disciple of Saint Beuno, and that he was sponsored by Cadfan, and is remembered in the St Cynhaearn's Church, Ynyscynhaearn.