The Alzette (Luxembourgish: Uelzecht, German: Alzig) is a river with a length of 73 kilometers in France and Luxembourg. It is a right tributary of the Sauer.
It rises in Thil near the town Villerupt in the Meurthe-et-Moselle département, France. It crosses the border with Luxembourg after a few kilometres. At Lameschmillen (near Bergem) it is joined by the Mess. It flows through the Luxembourgish towns Esch-sur-Alzette, Luxembourg City and Mersch, and empties into the Sauer near Ettelbruck.
The Sauer is a tributary to the Moselle.