William IX may refer to:
- William IX of Aquitaine (1071–1126)
- William IX, Count of Poitiers (1153–1156)
- William IX, Marquis of Montferrat (1494–1518)
- Wilhelm, Duke of Jülich-Cleves-Berg (1516–1592)
- William I, Elector of Hesse (or Hesse-Cassel), also known as William IX, Landgrave of Hesse-Kassel, (1743–1821)
See also
- King William (disambiguation)
- Prince William (disambiguation)
- List of people with given name William#Royalty and nobility