Niederfladnitz Schloss Niederfladnitz, Protected castle in Hardegg, Austria.

Schloss Niederfladnitz is a stately home located in Hardegg within the Waldviertel region of Lower Austria. The estate sits along the Thayatal Straße and forms part of the architectural character of the Hollabrunn District.

The castle was first documented in 1279 with Otto de Vlednitzer in connection to the site. By the 16th century, it had become an administrative hub under the Eyczing family's management.

The name Niederfladnitz derives from the medieval family name Vlednitzer, recorded in 1279 as Otto de Vlednitzer. The Czech name Dolní Blatnice is also used for the site. The castle is not open to the public; the municipality of Hardegg, which owns part of it, runs a kindergarten in the building.

Schloss Niederfladnitz stands on the B30 Thayatal Straße west of Retz in the Waldviertel, in the Hardegg municipality. The building is not open to the public and serves partly as a municipal facility, including a kindergarten, so it can only be viewed from the road outside.

During 1944 and 1945, Hungarian Jewish forced laborers worked on the estate for agricultural purposes. This period remains an important and somber part of the site's recent past.