The Hagener Au is a stream, roughly 10 km 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 50 m runs through a pipe. Together with the Passader See the Hagener Au is a protected Special Area of Conservation.