Seithenyn sometimes known as Seithenyn of the feeble mind is a figure from Welsh legend, apparently contemporary with King Gwyddno Garanhir. He is mentioned in a poem in the Llyfr Du Caerfyrddin, but becomes the protagonist of the story in a later version of the legend, in which he was responsible for the sea-defences of Cantre'r Gwaelod or Maes 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.