Heidenreichstein is a town in the district of Gmünd, in the Austrian state of Lower Austria. It lies in the northwestern part of the country, within the Waldviertel region near the border with the Czech Republic.
The town is best known for Heidenreichstein Castle, a well-preserved medieval water castle that ranks among the largest and most important of its kind in Austria. The surrounding landscape is characterized by forests, ponds, and granite formations typical of the Waldviertel.