Brychan Brycheiniog was a legendary 5th-century king of Brycheiniog in Mid Wales.

Brycheiniog was an independent kingdom in South Wales in the Early Middle Ages. It allied with the Mercian kingdom in the post Roman era, to stabilise and control a central (Marches) area key to dominance over central Proto-England to the east and the south Welsh kingdom of Deheubarth to the west. It was conquered and pacified by the Armorican Normans between 1088 and 1095, though it remained Welsh in character. It was transformed into the Lordship of Brecknock and later formed the southern and larger part of the historic county of Brecknockshire. To its south was the Kingdom of Morgannwg.