The Reuss (Swiss German: Rüüss) is a river in Switzerland. With a length of and a drainage basin of , it is the fourth largest river in Switzerland (after the Rhine, Aare and Rhône). The upper Reuss forms the main valley of the canton of Uri. The course of the lower Reuss runs from Lake Lucerne to the confluence with the Aare at Brugg and Windisch.
The Reuss is one of the four major rivers taking their source in the Gotthard region, along with the Rhine, Ticino and Rhône.
The Gotthardreuss rises in the Gotthard massif, emerging from Lago di Lucendro (reservoir built in 1947; elevation 2,131 m) in the canton of Ticino and passing into the canton of Uri below the Brigghubel (1,898 m).
The Furkareuss rises east of Furka Pass (2,429 m), early joined by the Blaubergbach (sourced by two mountain lakes on 2,649 m) and several other creeks sourced by still existing glaciers, such as Sidelengletscher (3,170 m), Tiefengletscher (3,285 m) in the north, and Muttengletscher (2,908 m) and Witenwassergletscher (2,805 m) in the south, and eventually forms the east-west valley called Urseren, passing Realp at half way through the valley at 1,540 m in eastern direction until its eastern end at Andermatt. This source is only around 5 km away from, and on the same mountain massif as the source of the Rhône, but will ultimately lead to the opposite side of the continent.
Gotthardreuss and Furkareuss join at Hospental (1,446 m). Downstream of Andermatt the Reuss joins at 1,429 m the village creek Unteralpreuss, finally turns to the north, and passes through the 3 km-long narrow and deep Schöllenen Gorge and under the legendary Devil's Bridge (1,120 m).
At Göschenen (1,057 m) it is joined by the Göschenerreuss, sourced by the glaciers Dammagletscher (3,430 m) and Flachensteinfirn (3,150 m) besides several others. From here it forms the main valley of the canton of Uri, the Urner Reusstal, passing below Wassen (840 m), Gurtnellen (720 m), Amsteg and Silenen (500 m), and through Erstfeld (460 m), past Attinghausen and Altdorf (440 m), joining the southernmost part of Lake Lucerne (the Urnersee) between Flüelen and Seedorf (434 m).