William I, Count of Luxembourg (1081–1131) was count of Luxembourg (1096–1131), in succession to his elder brother Henry III of Luxembourg. They were both sons of Conrad and Clementia of Aquitaine. William was the first of his family to use the title count of Luxembourg in his documents.

Like his predecessors, he was embroiled in debates with the archbishop of Trier Bruno in 1122 and 1127, leading to William's excommunication.

Around 1105 he married Mathilde or Luitgarde of Northeim, daughter of Kuno, count of Beichlingen, and had :

• Conrad II († 1136), count of Luxembourg
• William, count of Gleiberg, documented in 1131 and in 1158
• Liutgarde (1120 † 1170), married Henri II (1125 † 1211), count of Grandpré