William IX may refer to:
William IX of Aquitaine (1071–1126) 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