Inland Kingdom in southeastern Wales, in and around the valley of the upper River Usk; its lands are now, roughly speaking, the south of the county of Powys.
Its traditional founder was Brychan, the son of an Irish King and a Welsh princess, and the father, so it is said, of twelve sons and twenty-four daughters, many of whom became saints (possibly to compensate for the dubiousness of their birth). The Kingdom probably resulted from an Irish incursion or alliance, c.500.
Towards the end of the 10th century, its King acknowledged the supremacy of the King of Deheubarth. In 1093 the King of Deheubarth was killed in battle with Norman knights near the confluence of the Honddu and the Usk, with the result that the Kingdom of Brycheiniog became the Marcher lordship of Brecknock or Brecon. Its King held out for a while in the Eppynt, the northern hills of his former realm.