Myrddin Wyllt (—"Myrddin the Wild", , ) 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.
The nickname Myrddin Wyllt (the "wild" or "mad" Myrddin) was applied to the figure at some later time,<!--Tatlock (1943)--> by the cywyddwyr bards before 1500,<!--Guto'r Glyn (c. 1412 – c. 1493) --> and by Elis Gruffydd (1490–1552) in his chronicle.
Although Myrddin of Welsh legend was originally unconnected to King Arthur in earlier Welsh tradition, Myrddin was reinvented as Merlin, Arthurian court magician, by Geoffrey of Monmouth, and Myrddin became indistinguishable with Merlin in later Welsh literature. there are earlier attestations, e.g., an interpolation in the Black Book of Carmarthen (c. 1250), though written in a late hand, and usage by the cywyddwyr ("nobility of poetry") poets Guto'r Glyn (c. 1412 – c. 1493)