Plethyn is a Welsh language folk music group that was at their zenith between 1978 and 1995. The members are Roy Griffiths, Linda Healy and John Gittins. The three members originate from Maldwyn, in mid-Wales. Griffiths and Healy are brother and sister; Gittins was born on a neighbouring farm near Meifod.
They specialise in the close harmonies of the plygain (cy) tradition. Most of their songs are arrangements of traditional songs, but some of them are new, often including lyrics by Myrddin ap Dafydd. Elfed Lewys had a significant influence on the group when he came to the area as a minister. On earlier recordings they accompany themselves on guitar, mandolin, whistle and accordion; on later recordings they are joined by other instrumentalists.
Plethyn is also the Welsh word for "braid" or "bond".