or is a lake and reservoir in Hattfjelldal Municipality and Hemnes Municipality in Nordland county, Norway. It has been the site of human occupation since the Stone Age. Its area of 219 km2 makes it the second largest lake in Norway by surface area. Without the dam which has regulated the lake since 1957, it would be 190 km2 and the third largest lake in Norway. Its depth is 240 m, its volume is estimated at 15 km3, and its surface elevation is 374 m above sea level.

Røssvassholmen

The 17.39 km2 island Røssvassholmen is located in the southern part of the lake. it is the largest island in the lake. The highest point on the island reaches 206 m above the lake's surface (at top regulation height). The island is Norway's second largest lake-island. The largest island within a lake in Norway is Helgøya, located within the lake Mjøsa.