William of Hauteville (c. 1027-1080) was one of the younger sons of Tancred of Hauteville by his second wife Fressenda. He is usually called Willermus instead of Wilelmus in Latin annals and so is often called Guillerm instead of Guillaume in French. He left Normandy around 1053 with his elder half-brother Geoffrey and full brother Mauger. He participated in the Battle of Civitate in the years of his arrival and was received cordially by his half-brother Humphrey, the reigning count of Apulia.