Altenstadt is a municipality in the Weilheim-Schongau district, in Bavaria, Germany.

Of ancient Roman origins, the town was mostly abandonded in the 13th century.

The inhabitants moved to a more secure new settlemente located kilometers further 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.