The Reuss (Swiss Standard German: [ˈrɔʏs]; Swiss German: Rüüss) is a river in Switzerland. With a length of 164 kilometres (102 mi) and a drainage basin of 3,426 square kilometres (1,323 sq mi), 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).