William Plantagenet, otherwise known as William de Warenne, 6th Earl of Surrey (1166 - 1240), was the son of Hamelin de Warenne and Isabel, daughter of William de Warenne, 3rd Earl of Surrey. His father Hamelin granted him the manor of Appleby, in the County of Lincoln.

De Warenne was present at the coronation of king John on May the 27th in 1190. When Normandy was lost to the French in 1204 he lost his Norman holdings, (in 1202 he was lieutenant of Gascony), but king John of England recompensed him with Grantham and Stamford.

His first tenure of office as Lord Warden of the Cinque Ports begun in 1204, and lasted until 1206. He was also a Warden of Welch marches between 1208-1213.

William was one of the few barons who remained loyal to king John (who was his cousin) during the king's difficulties with the barons, when they sought for the French prince to assume the English throne, and is listed as one of those who advised John to accede to the Magna Carta. His allegiance only faltered a few times when the king's cause looked hopeless.

In March, 1217 he again demonstrated his loyalty to England by supporting the young king Henry III, he was also responsible for the establishment of the cathedral at Salisbury.

Between the years 1200-1208, and during 1213-1226 he was to serve as the sherriff of Wiltshire. In 1214 he was again appointed Lord Warden of the Cinque Ports.

He married Matilda, eldest daughter and later co-heiress of William Marshal, and widow of Hugh Bigod, 3rd Earl of Norfolk, thus becoming by marriage the earl of Salisbury. They had a son and a daughter. The son John succeded his father as earl, while the daughter, Isabel, married Hugh d'Aubigny , 5th Earl of Arundel.

William may also have had an earlier, childless marriage to another Matilda, daughter of William d'Aubigny, earl of Arundel.