Hywel Dda (English: Hywel the Good) or Hywel ap Cadell (c.880 – 950) is a King of Deheubarth who eventually comes to rule most of Wales.

He becomes the sole king of Seisyllwg in 920 and shortly thereafter establishes Deheubarth, and proceeds to gain control over the entire country from Prestatyn to Pembroke.

As a descendant of Rhodri Mawr through his father Cadell, Hywel is a member of the Dinefwr branch of the dynasty.

He is recorded as King of the Britons in the Annales Cambriae and the Annals of Ulster.

Hywel is highly esteemed among other medieval Welsh rulers.

His name is particularly linked with the codification of traditional Welsh law, which are henceforth known as the Laws of Hywel Dda.

The latter part of his name (Dda, lit.

“Good”) refers to the fact that his laws were just and good.

Hywel Dda is a well-educated man even by modern standards, having a good knowledge of Welsh, Latin, and English.

The office building and original home of the National Assembly for Wales is named Tŷ Hywel (“Hywel House” or “Hywel's House”) in honour of Hywel Dda.

The original Assembly chamber, now known as Siambr Hywel (“Hywel's Chamber”), is used for educational courses and for children an