William de Braose, Lord of Abergavenny (c. 1197 – May 2, 1230) was the son of Reginald de Braose by his first wife, Gracia de Briwere. The Welsh, who detested him, called him Gwilym Ddu (Black William). He succeeded his father in his various lordships in 1227, including Abergavenny and Builth.

He was married to Eva Marshall, daughter of the famous William Marshall. They had four daughters and coheiresses:
1. Isabella, wife of Prince Dafydd ap Llywelyn
2. Maud, wife of Roger Mortimer, 1st Baron Wigmore
3. Eleanor, wife of Humphrey de Bohun
4. Eve, wife of William de Cantelou

He was captured by Llywelyn the Great, ruler of most of Wales, in fighting in the commote of Ceri near Montgomery, in 1228. William was ransomed for the sum of £2,000 and then made an alliance with Llywelyn, arranging to marry his daughter Isabella to Llywelyn's only legitimate son Dafydd ap Llywelyn. However on a visit to Llywelyn during Easter 1230 connected with this marriage William was caught in Llywelyn's bedchamber with Llywelyn's wife, the Princess Joan. This affair may have begun while he was held as a prisoner there. The Chronicle of Ystrad Fflur entry for 1230 reads:
"In this year William de Breos the Younger, lord of Brycheiniog, was hanged by the Lord Llywelyn in Gwynedd, after he had been caught in Llywelyn's chamber with the king of England's daughter, Llywelyn's wife."

Llywelyn had William publicly hanged on May 2, 1230 at Crogen, near Bala, watched by a crowd of about 800 onlookers. Joan herself was later pardoned.