Rottnan

or is a long river in Norway and Sweden. The river starts south of the Hof Finnskog Church in Solør, Norway and discharges into lake Mellan-Fryken at Rottneros in Värmland, Sweden. The main river begins at the Rundtjern, but that lake is fed by several streams and small lakes on and surrounding the Evensbergklinten and Husuberget mountains. The highest source is the small lake Helveteskjelda, at an elevation of above sea level. The river passes through the municipalities of Åsnes and Grue in Innlandet county, Norway and through the municipalities of Torsby and Sunne in Värmland County, Sweden.

Rottnan used to have three waterfalls close to the mouth in Fryken. Rottnafallet was the last and tallest at height. The waterfalls were built in 1927 for hydroelectric power. The name Rottnan comes from the old Swedish word Rotn which means "the roaring".