William of Montferrat (died 1177), also called William Longsword (but not related to the other men of the same name), was the son of William III, marquis of Montferrat. He was the elder brother of Conrad and Boniface, two other notable sons of William III.

In 1176 William was chosen by Raymond III, count of Tripoli, and Baldwin IV, king of Jerusalem, to marry Baldwin's sister Sibylla. William also gained the County of Jaffa and Ascalon in the marriage. The marriage was short-lived however, as William died in 1177 while Sibylla was pregnant with the future king Baldwin V.

There were several other Williams from Montferrat:

• William I (990-1017)
• William II (1084-c.1101)
• William III (c.1136-1191)
• William IV (1207-1225)
• William V (1253-1290)
• William VI (1464-1483)
• William VII (1494-1518)

Another (possibly William III?) accompanied Louis VII of France's retinue on the Second Crusade. There was also a Dominican monk named William of Montferrat.