William I, Count of Boulogne

William of Blois (c. 1132/37; † October 11, 1159 ) was a Count of Boulogne and Mortain and Earl of Surrey out of the house Blois.

William was the third son of King Stephen of Blois and of the Countess Matilda of Boulogne. From his father he was 1148/49 married to Isabel de Warenne, William de Warenne after their father, 3rd Earl of Surrey, had fallen on the second crusade. Thus William was from the right of his wife even Earl of Surrey. After the death of his elder brother Eustace in 1153 William was not told by his father as the new heir to the English throne. In order to end the civil war against the House of Plantagenet, King Stephen recognized in the Treaty of Wallingford instead his rival Henry of Anjou as the rightful heir to.

William seems to have come to terms with his abdication in any case he would close after his father's death in 1154 the retinue of King Henry II at. In addition to the methods inherited from his brother in 1153 he received from the County of Boulogne new king in 1154, the Norman County Mortain as a fief. In England he held the castles of Pevensey and Norwich, which he however had to resign in 1157 to the king. 1159 Wilhelm received by the King in Carlisle the knighting and then accompanied him to the unsuccessful campaign of conquest against Toulouse. On his return to England William died in Poitou and was buried in Montmorillon.

Since Wilhelm had no children, the county of Mortain Henry II was drafted into the ducal domain Normandy. However, the County of Boulogne inherited his sister Mary, however, was abbess of the Abbey of Romsey. His widow married in 1164 Hamelin, a half-brother of the king.