The Attert (Luxembourgish: Atert) is a river flowing through Belgium and Luxembourg, left tributary of the Alzette. Its length is 38 kilometres (24 mi), of which 32 kilometres (20 mi) are in Luxembourg and 6 kilometres (4 mi) in Belgium. Its source is in Nobressart, north-west of Arlon, in the Belgian province Luxembourg. It flows into the river Alzette at Colmar-Berg.