Hernstein is a town in the Austrian state of Lower Austria. It lies in the district of Baden, within the federal state's industrial quarter region south of Vienna.
The town is best known for Hernstein Castle (Schloss Hernstein), a historic estate that has occupied a site in the area since the Middle Ages. In the nineteenth century the castle was rebuilt in a romantic historicist style, and it later served as a hunting residence associated with the imperial court. Since the mid-twentieth century the castle has housed a training academy for Austrian police officers.
The surrounding landscape is characterized by the wooded foothills of the Vienna Woods and the limestone ranges of the Gutenstein Alps, which rise to the south and west of the town. Hernstein is connected to neighbouring communities by regional roads and lies roughly 40 kilometres southwest of Vienna.