Brycheiniog is a small independent petty kingdom in South Wales in the Early Middle Ages.

It often acts as a buffer state between England to the east and the powerful south Welsh kingdom of Deheubarth to the west.

It is conquered and pacified by the Normans between 1088 and 1095, though it remains Welsh in character.

It is transformed into the Lordship of Brecon and later forms the southern and larger part of the historic county of Brecknockshire.

To its south is the Kingdom of Morgannwg.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).