William I of Hauteville (before 1010 – 1046), known as William Iron Arm, was a Norman adventurer who was the founder of the fortunes of the Hauteville family. One of twelve sons of Tancred of Hauteville, he journeyed to the Mezzogiorno with his younger brother Drogo in the first half of the eleventh century (c.1035), in response to requests for help made by fellow Normans under Rainulf Drengot, count of Aversa.