William de Valence, born Guillaume de Lusignan, was a French nobleman and knight who became important in English politics due to his relationship to King Henry III of England. He was heavily involved in the Second Barons' War, supporting the king and Prince Edward against the rebels led by Simon de Montfort. He took the name de Valence after his birthplace, the Cistercian abbey of Valence, near Lusignan in Poitou.

The Second Barons' War (1264–1267) was a civil war in England between the forces of a number of barons led by Simon de Montfort against the royalist forces of King Henry III, led initially by the king himself and later by his son, the future King Edward I. The barons sought to force the king to rule with a council of barons, rather than through his favourites. The war also involved a series of massacres of Jews by de Montfort's supporters, including his sons Henry and Simon, in attacks aimed at seizing and destroying evidence of baronial debts. To bolster the initial success of his baronial regime, de Montfort sought to broaden the social foundations of parliament by extending the franchise to the commons for the first time. However, after a rule of just over a year, de Montfort was killed by forces loyal to the king at the Battle of Evesham.