Mythical regional ruler of early Wales whose name appears in a number of disconnected stories. At the time of the flooding of Cantre'r Gwaelod, his kingdom was along Cardigan Bay. The name Caer Wyddno was given to rocks eight miles from Aberystwyth. In the Hanes Taliesin, the baby Taliesin floats into his fish weir between Aberystwyth and the River Dyfi and is rescued by Gwyddno's feckless son Elffin. Gwyddno is probably identical with the Gwyddneu who owns the basket providing inexhaustible supplies of food in Culhwch ac Olwen; Ysbaddaden Pencawr sends Culhwch to retrieve it for a wedding feast.
See Rachel Bromwich, Trioedd Ynys Prydain, rev. edn. (Cardiff, 1978), 397–400, 554.