Vefsn is a municipality in Nordland county, Norway. It is part of the Helgeland traditional region. The administrative centre of the municipality is the town of Mosjøen. Other villages include Drevvassbygda, Elsfjord, and Husvik.
The municipality of Vefsn was established on 1 January 1838 (see formannskapsdistrikt). In 1862, the eastern district (population: 921) was separated from Vefsn to become the new municipality of Hattfjelldal. This left Vefsn with 5,051 residents. In 1876, the town of Mosjøen (population: 379) was separated from Vefsn to become a separate municipality.
In 1927, the northern district was separated from Vefsn to become the new municipality of Drevja and the southern district was separated to become the new municipality of Grane. After the separations, Vefsn had 3,119 residents remaining. In 1939, a small area of Vefsn (population: 45) was transferred to Mosjøen.