Lake of Flatschach (German: Flatschacher See) is an Austrian lake at Flatschach near Feldkirchen in Kärnten in the State of Carinthia.

Since its inflow passes through a region heavily used for agriculture, the lake is heavily contaminated with nutrients. The lake itself is located in the 272 hectares (670 acres) protected area of 'Flatschacher See'.

The following species of fish are found in Lake Flatschacher: Carassius carassius, Carp, Cyprinus carpio, common rudd, Rutilus rutilus and Sander lucioperca.

• Flatschacher See (Kärntner Institut für Seenforschung) (in German)