Brycheiniog was a kingdom of south-east Wales which emerged in the 5th and lasted until the 11th century.
The kingdom of Brycheiniog was formed out of the old kingdom of Garth Madrun (believed to have been centred on Talgarth) in the mid-5th century and named after its founder, King Brychan. Brychan was a son of Prince Anlach, an Irish settler who had peacefully taken control of the area by marrying Marchel, the heiress of Garth Madrun. Tradition says that Brychan had an extremely large number of sons and daughters, many becoming saints in Wales and Cornwall. His eldest son, Rhain Dremrudd, founded a dynasty which ruled this realm until the mid-7th century. Another dynastic marriage then united Brycheiniog with the kingdom of Dyfed for three generations of monarchs. By the mid-8th century Brycheiniog had become independent again, but there were dynastic struggles with a younger branch of the royal family. In the 880s, King Elisedd ap Tewdr recognised King Alfred the Great of England as his overlord. The last king, Gruffydd ab Elisedd died in the mid-11th century, when his three sons became mere lords under the Anglo-Norman rule. It was later superseded by the county of Brecknockshire.
The main legacy of the kingdom of Brycheiniog is etymological. It has lent its name to Brecknockshire (Welsh: "Sir Frycheiniog", the Shire of Brycheiniog), and Brecon (known as "Aberhonddu" in Welsh).