Sir William de Tracy (died c. 1189) was a knight and the feudal baron of Bradninch, Devon, with caput at the manor of Bradninch near Exeter, and was lord of the manors (amongst very many others) of Toddington, Gloucestershire and of Moretonhampstead, Devon. He is notorious as one of the four knights who assassinated Thomas Becket, Archbishop of Canterbury, in December 1170.
His grandfather, William de Tracy (died c. 1136), was an illegitimate son of King Henry I and the king granted him the feudal barony of Bradninch, Devon, which had escheated to the crown from William Capra, listed in the Domesday Book of 1086 as holding that barony. William left one daughter and sole-heiress, Grace de Tracy, who married John de Sudeley, son of Harold de Mantes. They had two children: Ralph de Sudeley (d. 1192), the eldest, who became his father's heir, and this William "de Tracy", who inherited his mother's barony of Bradninch and assumed her family name in lieu of his patronymic. He became a knight of Gloucestershire and held the lands of his brother by service of one knight's fee.
William de Tracy appears in a charter of his older brother Ralph de Sudeley (died c. 1192) assigning the manor of Yanworth, near Cirencester, to Gloucester Abbey. Two of the witnesses to that charter lived on land held by the Normandy branch of the de Tracys, and two of the English witnesses had previously witnessed a charter to Barnstaple Priory in 1146 for Henry de Tracy, who had married a daughter of Juhel de Mayenne as permitted by Stephen, King of England. In 1166 William held one fee from his brother Ralph.