Tunnsjøen (Southern Sami: Dåtnejaevrie) is a lake in the municipalities of Røyrvik and Lierne in Nord-Trøndelag county, Norway. The 100.18-square-kilometre (38.68 sq mi) lake lies just south of the large lake Limingen, and just west of the border with Sweden. It is 358 metres (1,175 ft) above sea level and has a volume of 8.68 cubic kilometres (2.08 cu mi). The deepest part of the lake is 220 metres (720 ft) deep.