The Inn is a river in Switzerland, Austria and Germany. It is a right tributary of the Danube and is approximately 500km long.

The source is located in the Swiss Alps, near St. Moritz in the Engadin region, which is named after the river (The river is called "En" in Romansh). 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 passes the border to Bavaria near Kufstein.

On Bavarian territory the river runs northwards and passes the cities of Rosenheim, Wasserburg am Inn and Waldkraiburg; then it turns east, runs through Mühldorf and Neuötting and is enlarged by two major tributaries, the Alz and the Salzach.

From here to the Danube, it forms the border between Germany (Bavaria) and Austria (Upper Austria). Cities on this last section of the river are Simbach, Braunau and Schärding.

In the city of Passau the Inn finally enters the Danube (as does the Ilz river there). Although the Inn has a greater average flow than the Danube when they converge in Passau, and its watershed contains Piz Bernina, the highest point in the Danube watershed, the Inn is considered a tributary of the Danube, which has a longer length, drains a larger surface area, and has a more consistent flow.