is a town in the district of Hollabrunn in Lower Austria, Austria.
It is situated in the Waldviertel region on the river Thaya, directly on the border with the Czech Republic. The Thaya valley is protected as a national park, adjacent to the Podyjí National Park on the Czech shore. Since 1975 the municipality comprises the Katastralgemeinden of Hardegg, Felling, Heufurth, Mallersbach, Merkersdorf, Niederfladnitz, Pleissing, Riegersburg, Umlauf (unpopulated, is entirely located within the Thayatal national park and comprises a mere 12 fishermen’s huts), and Waschbach. As Hardegg proper has only 78 inhabitants, it is often referred to as Austria’s smallest town.