Reichenburg Castle, Medieval castle in Brestanica, Slovenia.

Reichenburg Castle is a hilltop fortress at the point where the Brestanica Creek meets the Sava River, near the town of Brestanica in Slovenia. It has thick stone walls and defensive towers, and today it houses museum collections, art exhibitions, a restaurant, and a café.

The 895 charter long cited as the castle's first mention was shown by research to be a forgery. After Hungarian raids destroyed the earlier stronghold, Archbishop Konrad I of Salzburg had it rebuilt after 1126; the Romanesque core dates from 1131–1147. It appears as "Reichenburch" in 1309, and the Reichenburg family held it from 1141 until 1570, when Hans Rajnpreht died without heirs.

The castle is known as the only Trappist monastery in Slovenia: the French Trappists who took over the estate in 1884 turned it into the Monastery of the Redeemer of Mary, which operated until the Nazis expelled them in 1941. Today the castle is run by the Krško Cultural Centre as a museum, event venue and wedding location.

The castle stands on a rock spur sixty metres above the confluence of the Brestanica stream and the Sava, above the village of Brestanica. It is managed by the Krško Cultural Centre together with the Krško City Museum, and the grounds also serve as an events and wedding venue, so parts of the complex may be occupied on such days.

During World War II, German occupiers used the castle as a deportation center for thousands of Slovenes sent to camps elsewhere. After the war it served as a women's prison before eventually opening to the public as a cultural venue.