The Inn (Latin: Aenus;[2] Romansh: En) is a river in Switzerland, Austria and Germany.

The name Inn is derived from the old Celtic words en and enios, meaning water.

The first written mention from the years 105 to 109 (Publii Corneli Taciti historiarium liber tertius) reads: "... Sextilius Felix... ad occupandam ripam Aeni fluminis, quod Raetos Noricosque interfluit, missus..." ("... Sextilius Felix was sent to capture the banks of the Inn, which flows between the Rhaetian people and the Noric people.

")[3] The river is also mentioned by other authors of the Roman Empire as Ainos (Greek) or Aenus (Latin).

[5] The source of the Inn is located in the Swiss Alps, west of St. Moritz in the Engadine region, which is named after the river (Romansh Engiadina; Latin vallis Eniatina).

It runs north-eastwards, entering Austria, and from Landeck eastwards through the Austrian state of Tyrol and its capital, Innsbruck (bridge over the Inn), and crosses the border into Bavaria near Kufstein.