William I may refer to:
Kings William the Conqueror (–1087), also known as William I, King of England William I of Sicily (died 1166) William I of Scotland (died 1214), known as William the Lion William I of the Netherlands and Luxembourg (1772–1843) William I of Bimbia () William I of Württemberg (1781–1864) Wilhelm I (1797–1888), King of Prussia and later German Emperor
Nobles
William I of Gascony (died 848) William I of Aquitaine (died 918) William I of Montferrat () William I of Normandy (–942) William I of Montpellier () William I of Provence (–993) William I Talvas (–after 1030), seigneur of Alençon William Iron Arm (before 1010–1046), Duke of Apulia William I, Bishop of Utrecht (died 1076) William I, Count of Nevers (–after 1083) William I, Count of Burgundy (1020–1087) William de Warenne, 1st Earl of Surrey (died 1088) William I of Cerdanya (1068–1095) William of Bures (died 1142), French crusader William I of Béarn (died 1173) William le Gros, 1st Earl of Albemarle (died 1179) William I of Geneva () William of Champlitte (died 1209), Prince of Achaea William I of Cagliari (–1214) William I, Lord of Douglas (died ) William I, Count of Holland (–1222) Uilleam I, Earl of Ross (died 1274) William I de la Roche (died 1287), Duke of Athens William I, Count of Hainaut (1286–1337) William I, Duke of Bavaria (1330–1389) William I of Guelders and Jülich (1364–1402) William I, Margrave of Meissen (1343–1407), known as William I, the one-eyed William I, Landgrave of Hesse (1466–1515) William I, Count of Nassau-Siegen (1487–1559) William the Silent (1533–1584), Prince of Orange Guglielmo I Gonzaga (1538–1587), Duke of Mantua William I, Elector of Hesse (1743–1821)
See also King William (disambiguation) Prince William (disambiguation)