Myrddin Wyllt (Welsh: [ˈmərðɪn ˈwɨɬt]—"Myrddin the Wild", Cornish: Merdhyn Gwyls, Breton: Marzhin Gouez) is a figure in medieval Welsh legend. In Middle Welsh poetry he is accounted a chief bard, the speaker of several poems in The Black Book of Carmarthen and The Red Book of Hergest. He is called Wyllt—"the Wild"—by Elis Gruffydd, and elsewhere Myrddin Emrys ("Ambrosius"), Merlinus Caledonensis ("of Caledonia") or Merlin Sylvestris ("of the woods").
Although his legend centres on a known Celtic theme, Myrddin's legend is rooted in history, for he is said to have gone mad after the Battle of Arfderydd (Arthuret) at which Rhydderch Hael of Strathclyde defeated the Brythonic king Gwenddoleu. According to the Annales Cambriae this took place in 573. Myrddin fled into the forest, lived with the beasts and received the gift of prophecy.