Numedalslågen is a river in Norway. It is considered to be the second longest river in Southern Norway. It is located in the Numedal valley which runs through the counties of Vestfold, Buskerud and Vestland. At 356 kilometres (221 mi) long, it is of the longest rivers in Norway.

Numedal is a valley and a traditional district in Eastern Norway located within the county of Buskerud. It traditionally includes the municipalities Flesberg, Nore og Uvdal and Rollag. Administratively, it now also includes Kongsberg.