William I, Count of Boulogne

William I of Boulogne (c. 1137–1159), was the second son of King Stephen of England and Matilda I of Boulogne. In 1148 he married Isabel de Warenne, Countess of Surrey. In 1153 Stephen agreed to recognize Henry Plantagenet as his heir instead of William's brother Eustace IV. After Eustace died, the Treaty of Wallingford ending the Anarchy left William Boulogne and the lands intended for both brothers.

Henry II initially allowed him to keep the earldom of Surrey through his wife. William was suspected of knowing of a Flemish plot to kill Henry and fled to Normandy. He died of disease near Toulouse on 11 October 1159, childless, and his sister Marie I succeeded him.