Domnall mac Causantín (Modern Gaelic: Dòmhnall mac Chòiseim), anglicised as Donald II (died 900) was King of the Picts or King of Scotland (Alba) in the late 9th century. He was the son of Constantine I (Causantín mac Cináeda). Donald is given the epithet Dásachtach, "the Madman", by the Prophecy of Berchán.