Lake Ossiach (German: Ossiacher See) is the third largest lake in the Austrian state of Carinthia, superseded only by Lake Wörth and Lake Millstatt.
It is situated in the southern Nock Mountains range of the Gurktal Alps along the road between Villach and Feldkirchen, at a height of 501 m (1,644 ft). It reaches a maximum depth of 52.6 m (173 ft), the surface area is about 10.8 km2 (4.2 sq mi).
Lake Ossiach is a dimictic lake with mixing periods in spring and in late autumn. In summer the waters reach 28 °C. Several uninhabited parts of the shore are protected as natural reserves.
There are five main villages surrounding the lake, all of them mainly dependent on summer tourism: Annenheim and Sattendorf, parts of the Treffen municipality, Bodensdorf and Steindorf, all located on the northern shore, and Ossiach with Ossiach Abbey, after which the lake is named.