The Alzette (French pronunciation: [alzɛt]; Luxembourgish: Uelzecht [ˈuəltsəɕt] ⓘ; German: Alzig [ˈaltsɪç]) is a river with a length of 73 kilometres (45 mi) in France and Luxembourg.

It rises in Thil near the town Villerupt in the Meurthe-et-Moselle département, France.

It flows through the Luxembourgish towns Esch-sur-Alzette, Luxembourg City and Mersch, and empties into the Sauer near Ettelbruck.

This name was given to the Casemates du Bock;[2] a honeycomb of tunnels which runs under the ruins of the Fortress of Luxembourg.

The fortress protected the city of Luxembourg for centuries until it began to be dismantled in 1867.