The Ill is a river of Alsace, in north-eastern France. It is a left-side tributary of the Rhine.

It flows from the Vosges mountains and then runs northward through Alsace, flowing parallel to the Rhine. The Ill passes through Mulhouse, Colmar and Sélestat before meeting with the Rhine at Strasbourg.

Flowing through the city of Strasbourg, the river forms part of the 17th century fortifications and passes through a series of locks and channels in the picturesque old town, including the Petite-France district, where its waters were once used to power mills and tanneries.