Lake Sfânta Ana (; ; ) is the only crater lake in Romania located in the volcanic crater of the Ciomad volcano of the Eastern Carpathians, near Tușnad in the Natural Reserve of Mohoș, Harghita County, Romania.
Palynology studies concluded that the history of Lake Saint Anne began about 9,800-8,800 years ago, at the stage of peat bog and shallow lake.
The depth of Saint Anne Lake is constantly decreasing. In 1867 it was 12 m, but now it is less than 7 m. In 2005, the maximum depth of the lake was 6.4 m and the sediment thickness was about 4 m. The lake is supplied exclusively from precipitation, therefore the degree of mineralization of the water is very low. The water purity approaches of that of distilled water, with only 0.0029 ml mineral
In winter, the lake is covered with a layer of ice of up to 1 m. The lake is part of the Mohos Nature Reserve.
Near the lake there is a Roman Catholic chapel dedicated to Saint Anne.
Swimming in the lake in general is prohibited since April 2018. It is available only for sporting events under special permits.