The Ill (all capitals: ILL) is a 72 km long tributary of the Rhine river in the western Austrian province 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 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.