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 largely in a natural state and, together with its wet meadows, forms a rich habitat.

The Kossau Valley is a designated nature reserve.