The White Ernz (Luxembourgish: Wäiss Iernz, French: Ernz blanche, German: Weiße Ernz) is a river flowing through Luxembourg, joining the Sauer at Reisdorf. It flows through the towns of Larochette, Medernach and Ermsdorf. The river valley also includes the notable feature of having a twin castle from the 14th century named Larochette Castle. It gives its name to the commune of Vallée de l'Ernz, formed in 2012 through the fusion of Medernach and Ermsdorf.

49°51′37″N 6°14′47″E / 49.86021°N 6.24652°E / 49.86021; 6.24652