Langvassåga is a river that flows out of the lake Langvatnet in the municipality of Rana in Nordland county, Norway. It flows in a southeastern direction, then it's joined by the river Røvassåga, and then continues as a tributary to the main river Ranelva, south of the village of Røssvoll. The Langvassåga catchment area covers about 1,114 square kilometres (430 sq mi).