The Lahn River is a 245.6-kilometer (152.6 mi)-long, right (or eastern) tributary of the Rhine River in Germany. Its course passes through the federal states of North Rhine-Westphalia (23.0 km), Hesse (165.6 km), and Rhineland-Palatinate (57.0 km).
It has its source in the Rothaargebirge, the highest part of the Sauerland. It meets the Rhine at Lahnstein, near Koblenz. Important cities along the Lahn include Marburg, Gießen, Wetzlar, Limburg an der Lahn, Weilburg and Bad Ems.
Tributaries to the Lahn include the Dill, the Weil and the Aar Rivers. The lower Lahn has many dams with locks, allowing regular shipping from its mouth up to Runkel. Riverboats are also used on a small section north of the dam in Gießen.