Mosjøen, also known as Mussere in Southern Sami, is a town located in Vefsn Municipality within the Nordland county of Norway. It serves as the administrative center for the municipality and covers an area of approximately 6.55 square kilometers. As of 2024, the town has a population of 10,059 residents.
Historically, Mosjøen is recognized as the oldest town in the Helgeland region, with only Bodø being older within Nordland county. The area was declared a ladested (town) in 1875 and functioned as a town-municipality until 1961, when it merged into Vefsn Municipality. It previously served as a garrison town and customs place before its urban area was officially re-declared a town in 1998.