The Iller (German: [ˈɪlɐ] ⓘ; ancient name Ilargus) is a river of Bavaria and Baden-Württemberg in Germany.

It is formed at the confluence of the rivers Breitach, Stillach and Trettach near Oberstdorf in the Allgäu region of the Alps, close to the Austrian border.

Between Lautrach near Memmingen and Ulm it forms the border between the two German States Bavaria and Baden-Württemberg for about 50 kilometres (31 mi).

It ranks as the seventh of Bavaria's rivers by water flow, with an average throughput of 75 cubic metres per second (2,600 cu ft/s) at Senden, a short distance upstream from the Danube.

A bicycle route follows the Iller, which is also a popular location for rafting and trekking.