William I, Count of Luxembourg (1081–1131) was Count of Luxembourg from 1096 until his death in 1131, succeeding his elder brother Henry III of Luxembourg. They were both sons of Conrad I and Clementia of Aquitaine. William was the first of his family to use the title Count of Luxembourg in his documents.
Like his predecessors, William was embroiled in debates with Bruno, the Archbishop of Trier, in 1122 and 1127, leading to William's excommunication.
Around 1105 he married Mathilde or Luitgarde of Northeim, daughter of Kuno, Count of Beichlingen, with whom he had three children:
Upon his death in 1131, William was succeeded as Count of Luxembourg by his eldest son Conrad II.