Kössen is located in a broad basin between the Chiemgauer Alps in the north and the Kaiser Mountains in the south of the confluence of Großache river, the Kohlenbach and the Weißenbach.
Its main tributaries, apart from its headstreams, are the Kitzbühler, Fieberbrunner and Reither Ache, shortly before Kössen the Schwarzlofer from Reit im Winkl, and in Kössen the Kohlenbach and Weißenbach from the lake of Walchsee.