Marsjøen is a lake in Folldal Municipality in Innlandet county, Norway. The 2.64-square-kilometre (1.02 sq mi) lake lies in the northern part of the municipality, about 7 kilometres (4.3 mi) east of the larger lake Fundin. There is a dam at the south end of the lake which is used to regulate the water height for use in the nearby Einunna and Savalen hydro-electric power stations.