William IX | |
|---|---|
 Portrait by Macrino d'Alba | |
| Marquis of Montferrat | |
| Reign | 1494–1518 |
| Predecessor | Boniface III Palaiologos |
| Successor | Boniface IV Palaiologos |
| Born | 10 August 1486 |
| Died | 4 October 1518 Trino |
| Noble family | Palaeologus-Montferrat |
| Spouse(s) | Anna d'Alençon |
| Issue | Maria Palaeologina Margaret Palaeologina Boniface IV of Montferrat |
| Father | Boniface III, Marquis of Montferrat |
| Mother | Maria of Serbia |
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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.