William I, Count of Boulogne

William I of Blois (c. 1137 – 11 October 1159) was Count of Boulogne (1153–1159) and Earl of Surrey jure uxoris (1153–1159). He was the third son of King Stephen of England and Countess Matilda I of Boulogne.

When his elder brother, Eustace IV of Boulogne, died in 1153, William was passed over in the succession to England. His father instead conceded the throne to the young Henry Plantagenet. The new King Henry was quite generous in his treatment of William of Blois, confirming his possession of the Earldom of Surrey jure uxoris (in right of his wife).

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 of Blois had married Isabel de Warenne, 4th Countess of Surrey, in 1148. They had no children before his death in 1159. He died of disease before the gates of Toulouse, and was buried at the Poitevin abbey of Montmorel. He was succeeded as Count of Boulogne by his sister, Mary I. His widow remarried to Hamelin Plantagenet.

Annals of Monte Fernando 1159: "Ob. Willelmus comes Bolonie."