The Ill (all capitals: ILL) is a 72-kilometre-long (45 mi) tributary of the Rhine in the western Austrian state of Vorarlberg.

It flows from the northern slopes of the Silvretta mountain range and then runs north-west through Vorarlberg. The Ill passes through the Montafon and Walgau [als; de; fr; nl] valleys and the town Feldkirch. It joins the river Rhine a few kilometers northwest of Feldkirch ("Illspitz"), at the border with Switzerland. The Ill has several dams with hydroelectric power stations.