William III (c. 970 – 1042) was the third Marquis of Montferrat and Count of Vado from 991 to his death. He was the eldest son and successor of Otto I. William I and II were the father and son, respectively, of Aleram, the first Marquis, but neither served as Marquis himself.
| William III, Marquis of Montferrat | |
|---|---|
| Marquis of Montferrat | |
| Reign | 991 – 1042 |
| Predecessor | Otto I |
| Successor | Otto II |
| Born | c. 972 |
| Died | 1042 (aged 69–70) |
| Spouse | Waza |
| Issue | Otto II Henry |
| House | Aleramici |
| Father | Otto I |