William I, Count of Boulogne

William I, also known as William of Blois, was born around 1137. He was the second son of Stephen, King of England, and Matilda I, Countess of Boulogne. In 1148, he married Isabel de Warenne, the 4th Countess of Surrey, and became Earl of Surrey jure uxoris in 1153.

After the death of his elder brother Eustace IV in 1153, William was passed over for the English throne in favor of Henry Plantagenet, as agreed in the Treaty of Wallingford. This treaty allowed William to inherit lands intended for both him and his brother, increasing his wealth. A plot against Henry's life was discovered in 1154, and William fled to Normandy, possibly due to his alleged involvement.

William I died of disease near Toulouse on 11 October 1159 and was buried at the Poitevin abbey of Montmorel. He was succeeded in his county by his sister Marie I.