Schloss Waldenstein, Wolfsberg, Castle in Waldenstein, Carinthia, Austria

Waldenstein Castle is a medieval fortress in a side valley of the Lavanttal, southwest of Wolfsberg in Carinthia, Austria. It consists of a solid residential tower and a baroque chapel tower, both of which shape the outline of the building on its hilltop.

Built around 1150 as a castle for the Diocese of Bamberg, Waldenstein was held from the 12th to the 17th century by the Ungnad von Sonnegg, who rebuilt it in Gothic style in the 15th century. In the 16th century it became a centre of the Reformation. After Simon Ungnad died in 1607, the estate passed to his granddaughter and then through several owners, including the Schönborn family from 1700 to 1754.

The castle chapel in the southeast, dedicated to Mater Dolorosa, has a baroque tower visible from outside. Waldenstein was a Reformation centre in the 16th century: Hans von Ungnad financed a printing press in Urach where Primož Trubar, creator of written Slovenian, translated Luther's writings. A press and Glagolitic, Cyrillic and Latin type are said to have been brought here, though their fate is unknown.

The castle can only be seen from outside, as the interior is not accessible due to the state of the building. The paths through the surrounding valley give a good view of the towers from different angles.

The castle is owned by Kärntner Montanindustrie GmbH, and in the 20th century its rooms housed mining families from the Waldensteiner Graben. Iron mica is still extracted there today for rust-protective paints. The building now stands empty and is decaying.