Seithenyn (sometimes spelt Seithennin) is a figure from Welsh legend, apparently contemporary with King Gwyddno Garanhir. He is the protagonist of a poem in the Llyfr Du Caerfyrddin (Black Book of Carmarthen). According to legend, he was responsible for the sea-defences of Cantre'r Gwaelod (English: The Lowland Hundred) or Maes Gwyddno (English: the Plain of Gwyddno), in the kingdom of the legendary Gwyddno Garanhir, but neglected them one night because of his drunkenness. Because of this neglect, the sea overran it.
Cantre'r Gwaelod is said to lie beneath the waters of Cardigan Bay off the coast of Ceredigion near Aberdyfi, Wales. Seithenyn (named in some later sources as being the son of Seithyn Saidi), was in charge of the embankment there, and as such, it was his failure to discharge his duties which led to its drowning. Seithenyn is also listed in the Triads of the Island of Britain as one of the Three Immortal Drunkards of the Isle of Britain.