The Numedalslågen river is a major river in Norway that runs through the Numedal region. The name "Numedalslågen" is made up of "Numedal" (the region) and "Lågen" (Norwegian for "river"). The river has a total length of about 352 kilometers and flows from its source in the Hardangervidda Mountains to its mouth in the Frierfjord, which is part of the Skagerrak.

Numedalslågen traverses various landscapes including plateaus, valleys and forests, resulting in a varied environment along the river. The surrounding area is often characterized by natural beauty and offers opportunities for outdoor activities such as fishing, canoeing and hiking.

The river has a rich history closely linked to human settlement and use of the region. Historically, the river was used to transport timber harvested from the surrounding forests. Later, hydroelectric power stations were built along the river to generate power. This use continues to this day as hydropower is an important source of energy in Norway.

Numedalslågen is also known for its fishing, especially salmon and trout fishing. The river provides habitat for various species of fish and is a popular spot for anglers. The scenic beauty, the historical importance and the diverse recreational opportunities make the Numedalslågen an important part of the Norwegian cultural landscape.