kingdom · British Isles

Kingdom of Seisyllwg

730 CE – 920 CE (approx.)

A Welsh kingdom of Ceredigion and Ystrad Tywi, named after King Seisyll, which merged with Dyfed to form Deheubarth.

Origins

Seisyllwg was created when Seisyll ap Clydog of Ceredigion added Ystrad Tywi, the Tywi valley, to his realm in the 8th century. The new territory was named after him. Little else is known of its early rulers beyond genealogies.

Rule and society

The kingdom covered much of later Cardiganshire and Carmarthenshire, a land of upland pasture and coastal farms with important churches such as Llanbadarn Fawr. Its last native king, Gwgon, drowned in 871, and it passed by marriage to Rhodri the Great of Gwynedd, whose son Cadell inherited it.

Decline and legacy

Cadell's son Hywel Dda united Seisyllwg with Dyfed around 920, creating the Kingdom of Deheubarth. The name Seisyllwg fell out of use, though Ceredigion persists as a county.

Few sources survive for this state; details are uncertain.

Lineage

Kingdom of Ceredigion 420 CE – 730 CEKingdom of Ystrad Tywi 450 CE – 730 CEKingdom of SeisyllwgKingdom of Deheubarth 920 CE – 1197 CE

Connections (1)

More states of British Isles →

Cite this page
APA
The Epoch Institute. (2026). Kingdom of Seisyllwg. In The Epoch Institute States Register. https://the-epoch-institute.org/state/kingdom-of-seisyllwg
MLA
“Kingdom of Seisyllwg.” The Epoch Institute States Register, The Epoch Institute, 2026, https://the-epoch-institute.org/state/kingdom-of-seisyllwg. Accessed .
Chicago
The Epoch Institute. “Kingdom of Seisyllwg.” The Epoch Institute States Register. 2026. https://the-epoch-institute.org/state/kingdom-of-seisyllwg.
Harvard
The Epoch Institute (2026) ‘Kingdom of Seisyllwg’, The Epoch Institute States Register. Available at: https://the-epoch-institute.org/state/kingdom-of-seisyllwg (Accessed: ).

Last reviewed October 2026Random state ↻