William IV (1765–1837; r. 1830–1837) was King of the United Kingdom of Great Britain and Ireland and concurrently King of Hanover.

William IV may also refer to:

• William IV, Duke of Aquitaine (937–994)
• William IV of Provence (died 1030)
• William IV of Weimar or William, Margrave of Meissen (died 1062)
• William IV of Montpellier (r. 1058–1068)
• William IV, Count of Toulouse (c. 1040–1094)
• William IV, Marquis of Montferrat (c. 1030–1100)
• William IV, Count of Nevers (c. 1130–1168)
• William IV of Forcalquier (1130–1208)
• William IV of Saint Omer (fl. 1157–1191)
• William IV, Count of Ponthieu (1179–1221)
• William IV, Count of Jülich (c. 1210–1278)
• William IV, Lord of Douglas (died 1333)
• William IV of Holland or William II, Count of Hainaut (1307–1345)
• William IV of Hainault or William II, Duke of Bavaria (1365–1417)
• William IV, Lord of Egmont (1412–1483)
• William IV, Duke of Brunswick-Lüneburg (c. 1425–1503)
• William IV, Duke of Jülich-Berg (1455–1511)
• William IV, Princely count of Henneberg-Schleusingen (1478–1559)
• William IV, Duke of Bavaria (1493–1550)
• William IV, Landgrave of Hesse-Kassel (1532–1592)
• William IV, Prince of Orange (1711–1751)
• William IV, Grand Duke of Luxembourg (1852–1912)

See also

• King William (disambiguation)
• Prince William (disambiguation)
• List of people named William#Royalty and nobility
• Guillaume IV (disambiguation), the French equivalent of William IV