Nelaug is a lake in the municipalities of Froland and Åmli in Agder county, Norway. The 10 kilometres (6.2 mi) lake is formed by a dam which regulates a hydroelectric power plant on the Nidelva river. The village of Nelaug is located near the dam in Froland municipality. The lake is located about 11 kilometres (6.8 mi) south of the village of Åmli, about 15 kilometres (9.3 mi) southeast of the village of Dølemo, and about 20 kilometres (12 mi) north of Blakstad.

The Old Norse form of the name must have been Niðlaug. The first element Nið is the old name of the river Nidelva and the last element is laug which means "lake".