Seisyllwg, also known as Seisilwig, was a medieval Welsh petty kingdom which existed from 680 to 920, covering Ceredigion and part of Carmarthenshire. It was named for the King of Ceredigion, Seisyll, and, in the 10th century, Seisilwig became the base of power for King Hywel Dda, who came to rule most of Wales. In 920, Hywel merged Seisilwig with the Kingdom of Dyfed to form Deheubarth.