Seisyllwg was a petty kingdom of medieval Wales. 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 covered the modern county of Ceredigion, part of Carmarthenshire, and the Gower Peninsula. It is evidently named after Seisyll, king of Ceridigion in the 7th or early 8th century, but it is unknown if he was directly responsible for its establishment. In the 10th century Seisyllwg became the center of power for Hywel Dda, who came to rule most of Wales. In 920 Hywel merged Seisyllwg with the Kingdom of Dyfed to form the new kingdom of Deheubarth.