Hywel Dda or Hywel ap Cadell (c.880 – 950) was a King of Deheubarth who eventually came to rule most of Wales. He became the sole king of Seisyllwg in 920 and shortly thereafter established Deheubarth, and proceeded to gain control over the entire country from Prestatyn to Pembroke. As a descendant of Rhodri Mawr through his father Cadell, Hywel was a member of the Dinefwr branch of the dynasty. He was recorded as King of the Britons in the Annales Cambriae and the Annals of Ulster.