Reginald de Braose, 9th Baron Abergavenny (born 1182) in Bramber, Sussex,(died June 1228 in Brecon) was the youngest son of William de Braose, 7th Baron Abergavenny and Matilda, also known as Maud, de St. Valery and Lady de Lahaie (born 1148-1155), who had produced three sons, William, Giles and Reginald. The de Braoses were loyal to King Richard I but grew in power under King John of England. The dynasty was in conflict with John of England towards the end of his reign and almost lost everything.

Reginald de Braose was a scion of the powerful Marcher family of de Braose, helped manage its survival and was also related by marriage to the Welsh Princes of Wales. He was born at Bramber in 1182.

He supported his brother Giles de Braose in his rebellions against King John. Both brothers were active against the King in the Baron's War. Neither was present at the signing of Magna Carta in June 1215 because at this time they were still rebels who refused to compromise.

King John acquiesced to Reginald's claims to the de Braose estates in Wales in May 1216. Reginald became Lord of Brecon, Abergavenny, Builth and held other Marcher Lordships but was also very much a vassal of the Welsh leader Llewelyn Fawr, Prince of Gwynedd who became his father-in-law in 1222 when Reginald married Llywelyn's daughter, Gwladus Ddu, known as Black Gwladys due to her raven hair.

Henry III restored Reginald to favour and the Bramber estates (confiscated by King John) in 1217.

At this seeming betrayal, Rhys and Owain, Reginald's Welsh nephews who were Princes of Deheubarth, were incensed and they took Builth, except the castle. Llywelyn Fawr also became angry and his forces besieged Brecon. Reginald eventually surrendered to Llewelyn and gave up Seinhenydd (Swansea).

By 1221 they were at war again, with Llewelyn again laying siege to Builth. The siege was relieved by King Henry III's forces. From this time on Llewelyn tended to support the claims of Reginald's nephew John de Braose concerning the de Braose lands in Wales.

Reginald was a witness to the re-issue of Magna Carta by King Henry III in 1225.

He died two or three years later in 1227 or 1228 in Brecon and was succeeded by his son, the ill-fated William de Braose, 10th Baron Abergavenny. He is buried at Brecon Cathedral. His daughter Matilda de Braose was the wife of Rhys Mechyll, Prince of Deheubarth.