Rhain ap Cadwgan (English: Regin son of Cadogan; died c. 740) was an 8th-century king of Dyfed and Brycheiniog in Wales of the Early Middle Ages.
[1] Rhain's great-grandfather, Cloten, had married the heiress of Brycheiniog, briefly uniting the two kingdoms.
During Rhain's rule, however, King Seisyll of Ceredigion invaded Dyfed and annexed its Cantref Ystrad Tywi (c. 710), cleaving apart Rhain's realm; Ceredigion and Ystrad Tywi were merged by Seisyll to form Seisyllwg.
[1] Sometime after this, the reduced Dyfed-Brycheiniog was briefly called Rhainwg,[1] but the detached parts could not be maintained as a unit, and Rhain's younger brother - Awst (English: Augustus) - was appointed as a governor of Brycheiniog.
[citation needed] Rhain's son, Tewdws succeeded him in Dyfed.