William I, Count of Boulogne

William I (c. 1137 – 11 October 1159) was the count of Boulogne and earl of Surrey jure uxoris from 1153 until his death. He was the second son of King Stephen of England, and Countess Matilda I of Boulogne.[1]

William married Isabel de Warenne, 4th Countess of Surrey, in 1148.[1] When his elder brother, Eustace IV of Boulogne, died in 1153, William was passed over in the succession to England. His father instead signed the Treaty of Wallingford, conceding the succession to Henry Plantagenet, son of his cousin and rival Empress Matilda. The intention was to end the Anarchy, a succession struggle between Stephen and Matilda of which both sides were growing weary.

Stephen died in 1154, and Henry initially allowed William of Blois to retain the earldom of Surrey jure uxoris (in right of his wife).[2] However, Gervase of Canterbury asserts a plot against Henry's life was discovered in 1154 among some Flemish mercenaries. The plan was to assassinate Henry in Canterbury, and allegedly William of Blois had knowledge of this plot or was in connivance with the mercenaries. Whatever the truth, Henry fled Canterbury and returned to Normandy.

William had no children. He died in 1159 of disease near Toulouse, and was buried at the Poitevin abbey of Montmorel. He was succeeded in his county by his sister Mary I. His widow remarried to Henry's half-brother Hamelin.