Fosnes was established as a municipality on 1 January 1838 (see formannskapsdistrikt). On 1 January 1871, the western district of Fosnes (population: 1,472) was separated to form the new municipality of Flatanger. This left Fosnes with 2,655 residents. On 1 January 1889, a small area of Fosnes (population: 61) was transferred to the neighboring municipality of Vikten. Then on 1 January 1913, the western part of Fosnes (population: 1,631) was separated to form the new municipality of Otterøy. This left the much smaller municipality with 1,107 residents. On 1 January 1964, the Finnanger area of Fosnes on the island of Otterøya (population: 116) was transferred to the new municipality of Namsos.