Wasserbillig (Luxembourgish: Waasserbëlleg [vaːsɐˈbələɕ] ^ⓘ) is a town in the commune of Mertert, in eastern Luxembourg. As of 2024^[update], Wasserbillig has 3,485 inhabitants,^[1] which makes it the largest town in Mertert. Wasserbillig is the administrative seat of the commune of Mertert.

Wasserbillig lies at the confluence of the rivers Moselle and Sauer, which form the border with Germany at the town. On the opposite side of the Moselle and linked by a car ferry lies Oberbillig, Germany; on the opposite side of the Sauer and linked by vehicle and rail bridges lies Wasserbilligerbrück, Germany.

Wasserbillig is the lowest settlement in Luxembourg, at 132 m (433 ft) above sea level.

Around 100 AD, there was already a town where Wasserbillig is situated which the Roman named Biliacum. This is also where the second part of Wasserbillig's name comes from (-billig from Biliacum). It was mainly a transloading harbour for goods coming down the Sauer or by cart. During the construction of the bridge over the Sauer in 1952, remnants of an old Roman bridge were found.^[2]

The town's railway station is on the line between Luxemburg City and the German border, connecting east to Trier.