The Kossau is a stream in the district of Plön in eastern Schleswig-Holstein, Germany. It drains the lake Tresdorfer See [ ceb; de; nl] and flows past Lütjenburg and through the Großer Binnensee before entering the Baltic Sea near Hohwacht. The stream was dammed near Rantzau Castle [ da; de; sv] in the Middle Ages to form a lake which existed until the 18th century. It is about 22 kilometres (14 mi) long and most of its course consists of meanders. It is largely in a natural state and, together with its wet meadows, forms a rich habitat. The Kossau Valley is a designated nature reserve.

• List of rivers of Schleswig-Holstein

• Holstein Switzerland Nature Park (in German)