Donnchad mac Crinain (Modern Gaelic: Donnchadh mac Crìonain; anglicised as Duncan I, and nicknamed An t-Ilgarach, "the Diseased" or "the Sick"; ca. 1001 – 14 August 1040) was king of Scotland (Alba) from 1034 to 1040. He is the historical basis of the "King Duncan" in Shakespeare's play Macbeth.