One of the Medieval kingdoms of Wales.
Seisyllwg essentially comprised the kingdom of Ceredigion, with the addition of the territory of Ystrad Tywi.
Named after its first king, Seisyll who wrested control of Ystrad Tywi away from the neighbouring kingdom of Dyfed sometime in the mid to late eighth century.
The succession of kings for Seisyllwg is as follows;
- Subsumed within the kingdom of Deheubarth from 920
Most of the dates given for the reigns of Seisyll and his descendents are educated guesses. The only roughly secure dates we have are for the deaths of Arthwyr (or Arthen) and Gwgan derived from the Annales Cambriae.
Gwgan ap Meurig was the last independent king of the house of Seisyll; from 872 Seisyllwg became subsumed within the greater Gwynedd of Rhodri Fawr. On Rhodri's death in 878, Seisyllwg passed to his son Cadell ap Rhodri, although exactly how much autonomy he exercised over the kingdom is unclear.
On the death of Cadell ap Rhodri in 909, the kingdom passed into the hands of his two sons Hywel, and Clydog. On the latter's death in 920, Hywel, better known as Hywel Dda, united Seisyllwg with the kingdom of Dyfed and renamed the new combination as Deheubarth.