The Leutascher Ache (also: Leutasch, in its upper range Gaistalbach) is a river of Tyrol, Austria and of Bavaria, Germany, a left tributary of the Isar.

The Leutascher Ache springs in the Mieming Range in the region of Ehrwald in Tyrol and flows firstly in northern direction. After about 1 kilometre (0.62 mi), it takes up the water of the lake Igelsee [ceb; de; sv] and then flows between the Wetterstein mountains and the Mieming Range through the valley Gaistal [de; nl; ro] eastwards. Near Leutasch, it swings northeastwards and runs through the Leutaschtal [de], a high valley. After passing through the Leutasch Gorge, it discharges into the Isar in Bavaria near Mittenwald.