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. Some of the notable villages in Vefsn include Drevvassbygda, Elsfjord, and Husvik.

Helgeland is the most southerly district in Northern Norway. Generally speaking, Helgeland refers to the part of Nordland county that is located south of the Arctic Circle. It is bordered in the north by the Saltfjellet mountains and Svartisen glacier, which form a natural border with the Salten district. In the south, Helgeland borders Trøndelag county.