William Longsword may refer to:
- William I, Duke of Normandy (893-942), second duke of Normandy
- William of Winchester, Lord of Lunenburg
- William of Montferrat, Count of Jaffa and Ascalon (c. 1140-1177), crusader
- William Longespée, 3rd Earl of Salisbury (1176–1226), illegitimate son of Henry II, loyal to King John