Tunnsjøen
or  is a lake in the municipalities of Røyrvik and Lierne in Trøndelag county, Norway.  The  lake lies just south of the large lake Limingen, and just west of the border with Sweden. It is  above sea level and has a volume of .  The deepest part of the lake is  deep.  It is the seventh largest lake in Norway.

Gudfjelløya
The island Gudfjelløya () lies in Tunnsjøen lake. It has a high point of  above sea level, which is  higher than the lake.  This makes Gudfjelløya the highest island on a lake in Norway.