| Hagener Au | |
| Subdivision Type1: | Country |
| Subdivision Name1: | Germany |
| Subdivision Type2: | State |
| Subdivision Name2: | Schleswig-Holstein |
| Subdivision Type3: | District |
| Subdivision Name3: | Plön |
| Subdivision Type4: | Reference no. |
| Subdivision Name4: | DE: 965529 |
| Length: | 10 km |
| Source1 Location: | Source: Passader See |
| Mouth Location: | Kiel Fjord between Laboe and Stein |
| Basin Landmarks: | Villages: Probsteierhagen, Lutterbek, Stein |
| Custom Label: | References |
The Hagener Au (pronounced as /de/) is a stream, roughly 10km (10miles) long, in the district of Plön in the north German state of Schleswig-Holstein. It is an outlet of the Passader See. From the lake's southwestern bay the stream flows in a northerly direction and discharges between Laboe and Stein in the Baltic Sea.
Sections of the Hagener Au are very natural and surrounded by wetlands. In addition the stream is a habitat for the rare spined loach. The stream has been straightened near its mouth and the last 50m (160feet) runs through a pipe.Together with the Passader See the Hagener Au is a protected Special Area of Conservation.