William IX Palaeologus (10 August 1486 – 4 October 1518) was Marquis of Montferrat from 1494 until his death.

He was a member of the House of Palaeologus-Montferrat, a cadet branch of the Palaiologos dynasty which had once ruled the Byzantine Empire.

During the Italian Wars, in 1513, he protected the French retreat from Milan; however, to avoid retaliation 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.

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.