Røssvatnet is a lake and reservoir in the municipalities of Hattfjelldal and Hemnes in Nordland county, Norway. It has been the site of human occupation since the Stone Age. Its area of makes it the second largest lake in Norway by surface area. Without the dam which has regulated the lake since 1957, it would be and the third largest lake in Norway. Its depth is , its volume is estimated at about , and its surface elevation is above sea level.