The Sauer (German, Luxembourgish) or Sûre (French) is a river in Belgium, Luxembourg and Germany. A left tributary of the river Moselle, its total length is 173 km.
Rising near Vaux-sur-Sûre in the Ardennes in southeastern Belgium, the Sauer flows eastwards and crosses the border with Luxembourg near Martelange. It forms the border between Belgium and Luxembourg for 13 km north of Martelange. West of Esch-sur-Sûre it flows into an artificial lake, the Upper Sûre Lake, which gives its (French) name to the Luxembourgian commune of Lac de la Haute-Sûre. After flowing through Ettelbruck and Diekirch, the Sauer forms the border between Luxembourg and Germany for the last 50 km of its course, passing Echternach before emptying into the Moselle in Wasserbillig. The rivers Wiltz, Alzette, White Ernz, Black Ernz, Our, and Prüm are tributaries.