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.

Deheubarth was a regional name for the realms of south Wales, particularly as opposed to Gwynedd. It is now used as a shorthand for the various realms united under the House of Dinefwr, but that Deheubarth itself was not considered a proper kingdom on the model of Gwynedd, Powys, or Dyfed is shown by its rendering in Latin as dextralis pars or as Britonnes dexterales and not as a named land. In the oldest British writers, Deheubarth was used for all of modern Wales to distinguish it from Hen Ogledd, the northern lands whence Cunedda originated.