Mjøsundvatnet is a lake in Namsos Municipality and Nærøysund Municipality in Trøndelag county, Norway. The 8.76-square-kilometre (3.38 sq mi) lake lies in the far northern part of Namsos, and it extends a short distance into Nærøysund on both ends. The lake is accessible by road from Salsbruket in Nærøysund Municipality.