Altenstein Castle, Medieval castle ruins in Maroldsweisach, Germany.

Altenstein Castle is a ruined hilltop fortress on a narrow ridge near Maroldsweisach, in the Haßberge hills of Bavaria. The surviving stone walls enclose an inner ward, and the elevated position offers open views over the Weisachtal valley below.

Altenstein is first recorded indirectly in 1225, when a document names Marquard Magnus de antiquo lapide; in 1231 it appears directly as a castrum with an ecclesia. By 1296 eight families lived there jointly, and a 1441 Burgfrieden named ten brothers and cousins from five families. The Steins moved down to Pfaffendorf in 1703, and the castle was donated to Haßberge district in 1972.

The castle's name comes from the lords of Stein zu Altenstein, whose line died out in 1875 with Karl vom Stein zum Altenstein. The site is one of the largest castle ruins in Franconia, and since 2003 the renovated walls have been open for visits; the gatehouse of the outer ward, built in the 17th or 18th century, is used again as a residence.

The ruin is not freely accessible: an entrance fee is charged, and the site was made safe and opened up for visitors during the renovation completed in 2003. The gatehouse of the outer ward, built in the 17th or 18th century, is used again as a residence and forms the entrance to the site. The castle hill rises about 150 metres above the Weisach valley.

The ridge where the ruins stand was once part of a network of fortifications, several of which are still visible today as separate structures. These remains are now connected by a 14-station castle trail that runs through the wider region.