Mosjøen or Mussere (Southern Sami) 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 Municipality, losing its status as a town (ladested). It is also a former garrison town and customs place. After a change in law during the 1990s, the urban area of Mosjøen was declared to be a town once again in 1998. People from Mosjøen are referred to using the demonym "mosjøværing". The 6.55-square-kilometre (1,620-acre) town has a population (2024) of 10,059 and a population density of 1,536 inhabitants per square kilometre (3,980/sq mi).