Seisyllwg was a kingdom of medieval South Wales.

Origins
Seisyll (born c. 665) was a direct descendant of King Ceredig's eldest son, Usai. He was a late 7th century king of Ceredigion who invaded Dyfed and managed to conquer Ystrad Tywi. His enlarged kingdom of Ceredigion and Ystrad Tywi became known as Seisyllwg in his honour.

Expansion and Merger with Dyfed
By 800, the kingdoms of Builth and Gwerthrynion were united through intermarriage and conquest with Seisyllwg.

The kingdom passed to Rhodri Mawr, the King of Gwynedd, in 872, and was from then onwards ruled by a branch of his family. In 920, Hywel Dda merged Seisyllwg with Dyfed to create the Kingdom of Deheubarth. From this power base, he later went on to unite almost all of Wales.

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