Sierndorf is a town in the district of Korneuburg, in the Austrian state of Lower Austria. It lies in the northeastern part of the country, within the Weinviertel region north of Vienna.
The municipality is made up of several settlements, or Katastralgemeinden, and is centred on the main town of Sierndorf. The surrounding landscape is gently rolling farmland typical of the Weinviertel, and the area has long been shaped by agriculture.
Among its notable features is Sierndorf Castle (Schloss Sierndorf), a historic estate that has stood in the town for centuries and remains a local landmark. The town also has a railway station on the line connecting Vienna with the Korneuburg area, giving it direct links to the capital.