Schwarzenau is a market town in the District of Zwettl in the Austrian state of Lower Austria. It lies in the northwestern part of the state, in the Waldviertel region near the Czech border.
The town is situated on the German Thaya, a river that forms part of the border between Austria and the Czech Republic. Its location on the river has shaped the settlement's character as a small market community in a rural, wooded part of Lower Austria.
As a market town (Marktgemeinde), Schwarzenau serves as a local administrative and service centre for the surrounding area. It is one of several small towns within the Zwettl district, a largely agricultural and forested region of Lower Austria.