William I de Moyon (d. post 1090) (alias de Moion, also de Mohun), 1st feudal baron of Dunster in Somerset, was seigneur of Moyon in Normandy and became Sheriff of Somerset in 1086. He founded the English de Mohun family in the Westcountry. Recorded in the Domesday Book of 1086 as a tenant-in-chief of William the Conqueror holding a number of manors in Somerset with caput at Dunster Castle.