The Hagener Au (German pronunciation: [ˈhaːɡənɐ ˈaʊ̯] ⓘ) is a stream, roughly 10 kilometres (6.2 mi) long, in the district of Plön in the north German state of Schleswig-Holstein.

From the lake's southwestern bay the stream flows in a northerly direction and discharges between Laboe and Stein in the Baltic Sea.

The stream has been straightened near its mouth and the last 50 metres (160 ft) runs through a pipe.

Together with the Passader See the Hagener Au is a protected Special Area of Conservation.

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