William IX (10 August 1486 - 1518) was marquess of Montferrat from 1494 until his death.
He was the son of marquis Boniface III and Maria Branković.
After succeeding his Boniface at his death (1494), he continued the latter's pro-French policy and, in 1508, he married to princess Anna d'Alençon, daughter of René d'Alençon. During the Italian Wars, in 1513, he protected the French retreat from Milan; however, to avoid retaliations from Maximilian Sforza William was forced to pay him 30,000 scudi.
The Milanese Duke did not respect the treaty, and his troops invaded Montferrat and sacked numerous cities. At the same time, informed that his relative Oddone, marquess of Incisa, was trying to obtain the title of marquess of Montferrat, William marched to Incisa and occupied that lordship. Oddone and his son Badone were sentenced to death. However, the annexation was later declared null by Holy Roman Empire diplomacy, but in the subsequent year William was able to maintain the new lands.
He died in 1518 and was succeeded by his son Boniface IV, initially under with his mother Anne d'Alençon as tutor.
| Preceded by Boniface III |
Marquess of Montferrat 1494–1518 |
Succeeded by Boniface IV |