The Inn (German pronunciation: [ɪn] ; Latin: Aenus; Romansh: En) is a river in Switzerland, Austria and Germany. The 518 km (322 mi) long river is a right tributary of the Danube, being the third largest tributary of the Danube by discharge. The highest point of its drainage basin is the summit of Piz Bernina at 4,049 m (13,284 ft). The Engadine, the valley of the En, is the only Swiss valley whose waters end up in the Black Sea (via the Danube).

The name Inn is derived from the old Celtic words en and enios, meaning water. In a document of 1338, the river was named Wasser (German for water).