Domnall mac Causantín (Modern Gaelic: Dòmhnall mac Chòiseim, IPA:[ˈt̪oːvnəɫ̪ˈmaʰkˈxoːʃɪm]), anglicised as Donald II (c. 30 June 862 – April 900), was King of the Picts or King of 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.
Donald usurped the throne from his cousin, Giric in 889 who killed his uncle Àed Mac Cináeda (r. 877– 879). He continuously fought the Vikings in The North, winning victories but would be killed in 900 at war, possibly against King Harald Fairhair. He was succeeded by his cousin Caustantín Mac Àeda (Constantine II)