William III of Montferrat

William III (c. 970–1042) was the third Marquis of Montferrat from 991, succeeding his father Otto I. He founded Spigno monastery and made donations to regional abbeys, but abandoned the imperial cause, allying with Ulric Manfred II of Turin and Bishop Leo of Vercelli. The allies soon quarrelled, and after Leo besieged him at Santhià, William burned Vercelli in revenge.

Peace with Manfred was sealed by marrying his son Henry to Manfred’s daughter Adelaide. He resisted Conrad II until the emperor destroyed his Orba valley fortress, and dying in 1042 he left the marquisate to Otto II.