Mosjøen is a town in Vefsn Municipality in Nordland county, Norway. Mosjøen is the oldest town in the Helgeland region, with only the town of Bodø being older within Nordland county. The town is also the administrative centre of Vefsn Municipality. The old village of Mosjøen was declared a ladested in 1875. It was also a town municipality from 1875 until 1961 when it was merged into Vefsn, losing its status as a town (ladested). It is also a former garrison town and customs place. In 1998, the urban area of Mosjøen was declared to be a town once again. People from Mosjøen are referred to using the demonym "mosjøværing". The 6.36-square-kilometre (1,570-acre) town has a population (2018) of 9,834 and a population density of 1,528 inhabitants per square kilometre (3,960/sq mi).

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.