The Lech (Latin: Licus, Licca) is a river in Austria and Germany. It is a right tributary of the Danube 255 kilometres (158 mi) in length with a drainage basin of 3,919 square kilometres (1,513 sq mi). Its average discharge at the mouth is 115 m³/s (4,100 cu ft/s). Its source is located in the Austrian state of Vorarlberg, where the river rises from lake Formarinsee in the Alps at an altitude of 1,870 metres (6,120 ft). It flows in a north-north-easterly direction and crosses the German border, forming the Lechfall, a 12-metre-high (39 ft) waterfall; afterwards the river enters a narrow gorge (the Lechschlucht). Leaving the Alps, it enters the plains of the Allgäu at Füssen at an elevation of 790 metres (2,580 ft) in the German state of Bavaria, where it used to be the location of the boundary with Swabia. The river runs through the city of Füssen and through the Forggensee, a man-made lake which is drained in winter. Here, it forms rapids and a waterfall.

The river flows further northwards through a region called the Lechrain, and passes the cities of Schongau, Landsberg, Augsburg (where it receives the Wertach) and Rain before entering the Danube just below Donauwörth at an elevation of 410 metres (1,330 ft). It is not navigable, owing to its torrential character and the gravel beds which choke its channel. There are extensive views of the Lech valley from Neuschwanstein Castle, near Füssen.