Røssvatnet (Southern Sami: Reevhtse) 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 219 square kilometres (85 sq mi) 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 square kilometres (73 sq mi) and the third largest lake in Norway.