First husband of Sibyl, sister to King Baldwin IV of Jerusalem, and father of the infant king Baldwin V (1183-1186).
The eldest son of William V “the Old,” marquis of Montferrat, and Judith of Austria, William Lungaspada (Longsword) was chosen in 1176 by Baldwin IV and the High Court of the kingdom of Jerusalem as husband for Sibyl, the king’s elder sister and heir, most probably on account of his family’s ties with Frederick I Barbarossa, Holy Roman Emperor, from whom the kingdom hoped to receive assistance.
On his marriage (November 1176), William was invested with the county of Jaffa (mod. Tel Aviv-Yafo, Israel) and Ascalon (mod. Tel Ashqelon, Israel). He was the obvious candidate to be regent of the kingdom in the event of the leper Baldwin IV becoming incapable of exercising government, and eventually to become king himself. However, William fell ill and died in June 1177. The following winter Sibyl gave birth to a posthumous son, Baldwin V, who was crowned co-king with Baldwin IV in 1183, and succeeded as sole ruler in 1185, but died the following year.