Seisyll ap Clydog was

King of Ceredigion Prior to the Conquest of Wales, completed in 1282, Wales consisted of a number of independent kingdoms, the most important being Gwynedd, Powys, Deheubarth (originally Ceredigion, Seisyllwg and Dyfed) and Morgannwg (Glywysing and Gwent). Bounda ...

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

some time in the late 7th or early 8th century. He gave his name to the later kingdom of

Seisyllwg Seisyllwg () was a petty kingdom of medieval Wales.Davies, p. 85 It is unclear when it emerged as a distinct unit, but according to later sources it consisted of the former Kingdom of Ceredigion plus the region known as Ystrad Tywi. Thus it covere ...

, which consisted of Ceredigion plus the region known as

Ystrad Tywi Ystrad Tywi (, ''Valley of the Tywi'') is a region of southwest Wales situated on the banks of the River Tywi and possibly the River Loughor. Although Ystrad Tywi was never a kingdom itself, it was historically a valuable territory and was fough ...

; as such he was possibly the king responsible for the expansion. However, there is no contemporary evidence of this, and almost nothing is known of his life or reign.

History

Seisyll appears in the

Harleian genealogies __NOTOC__ The Harleian genealogies are a collection of Old Welsh genealogies preserved in British Library, Harley MS 3859. Part of the Harleian Library, the manuscript, which also contains the ''Annales Cambriae'' (Recension A) and a version of th ...

for the kings of Ceredigion. Here, he is named as the son of Clydog or Clitauc Artgloys, and is the sixth in descent from

Ceredig Ceredig ap Cunedda (died 453), was king of Ceredigion in Wales.