Sonnenstein Castle, Medieval castle and stately home in Pirna, Germany.

Sonnenstein Castle stands on a rock plateau about 70 metres above the Elbe in Pirna. The complex combines a tall pale-yellow block with a red roof, dormers and a corner turret, a long lower wing with five dormers fronted by tall vertical columns, and a section with a red domed roof and small spire. Rough stone retaining walls support the raised base.

First documented as Castrum Pirne on 5 December 1269, the site began as a Slavonic settlement and fortification above the Elbe. In 1372 Emperor Charles IV signed the Treaty of Pirna here, and from 1570 to 1573 Rochus zu Lynar and Hans Irmisch built a fortress with three towers and several bastions. It opened as a mental asylum on 18 July 1811; between 1940 and 1941 more than 13,000 patients were murdered in the Aktion T4 killing centre.

The Pirna-Sonnenstein memorial, opened on 9 June 2000 in the former men's building C 16, documents the murder of more than 13,000 patients between 1940 and 1941. It is run by the Stiftung Sächsische Gedenkstätten. The name Sonnenstein refers to the rock plateau above the Elbe, and the site is still used today by a workshop for disabled people opened in 1991.

The complex sits on a rock plateau about 70 metres above the Elbe, reached on foot from Pirna's old town. The memorial centre for the Aktion T4 victims, opened on 9 June 2000, is managed by the Stiftung Sächsische Gedenkstätten. Since December 2011 the buildings have housed the district administration of Sächsische Schweiz-Osterzgebirge, so some parts are offices rather than visitor areas.

In 1639, commander Johann Siegmund von Liebenau held Sonnenstein against Johan Banér's Swedish troops during a months-long siege, though artillery fire badly damaged the buildings. Liebenau also pushed through the separation of the fortress's military role from the district administration, which moved to a building on Pirna's market square in 1674.