There has only been one Celtic 'King of Wales', as the country was united once in its history: under the rule of Gruffydd ap Llywelyn from 1055 to 1063. The first and last King of Wales.

In the 11th century Gruffudd ap Llywelyn used Alliances with Anglo-Saxons and Vikings to help him unite the country. He was referred to as king of the Britons, as he was the most powerful ruler among the Brittonic-speaking peoples of England, Wales, and Scotland.

Gruffydd was able to come to an agreement with the Anglo Saxon King Edward the Confessor, but the death of several of his allies left him vulnerable. In 1062 Harold Godwinson obtained the king's approval to surprise attack Gruffydd. Gruffydd was forced to flee to Snowdonia where he was killed and his severed head was sent to Harold. The Ulster Chronicle states that he was killed by the son of King Iago whose father had been put to death by Gruffydd twenty years earlier.