William I may refer to:

Kings

• William the Conqueror (c. 1028–1087), 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 (fl. 1850s)
• William I of Württemberg (1781–1864)
• William I, German Emperor (1797–1888)

Nobles

• William I of Gascony (died 848)
• William I of Aquitaine (died 918)
• William I of Montferrat (fl. 924)
• William I of Normandy (c. 900–942)
• William I of Montpellier (fl. 980s)
• William I of Provence (c. 950–993)
• William I Talvas (c. 995–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 (c. 1030–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 I 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 (fl. 1190s)
• William of Champlitte (died 1209), Prince of Achaea
• William I of Cagliari (c. 1160–1214)
• William I, Lord of Douglas (died c. 1214)
• William I, Count of Holland (c. 1167–1222)
• 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)