Altenstadt () is a municipality in the Weilheim-Schongau district, in Bavaria, Germany.
Of ancient Roman origins, the town was mostly abandoned in the 13th century. The inhabitants moved to a more secure new settlement located kilometers further away at the River Lech, also named Schongau. The old town was rechristened "Altenstadt", meaning "Old City" in German.
A significant sight is the Romanesque basilica minor of St. Michael with remains of medieval wall painting and several examples of Romanesque sculpture.