Traunsee (, ) is a lake in the Salzkammergut, Upper Austria, Austria. Its surface is approximately 24.5 km<sup>2</sup> and its maximum depth of 191 metres makes it the deepest and by volume largest lake located entirely within Austrian territory; only Lake Constance on the border is deeper and bigger. It is a popular tourist destination, and its attractions include Schloss Ort, a medieval castle.
At the North end of the lake is Gmunden, at the south end is Ebensee. The lake is surrounded by mountains, including the Traunstein, and a number of other towns and villages surround the lake, including Altmünster and Traunkirchen.
There is a local legend that speaks of a waterhorse that lives in the lake. Records mention a mermaid riding on the back of a creature who lives in the waters. Locals refer to it as "Lungy" and photographs have been taken of the creature.