William fitz Duncan (a modern anglicisation of the Old French Guillaume fils de Duncan and the Middle Irish Uilleam mac Donnchada) was a Scottish prince, the son of King Duncan II of Scotland by his wife Ethelreda of Dunbar. He was a territorial magnate in northern Scotland and northern England and a military leader. William fitz Duncan died in 1147, whereupon Moray reverted back into the hands of David.