Heiliger See (English: Holy Lake) is a lake within the city limits of Potsdam, Brandenburg, Germany, located northeast of the city center and bordering the historic park known as the New Garden.

The lake is 1.33 km long and 300 metres wide on average.

On the north shore there are informal places for swimming and sunbathing, as well as a connection to Lake Jungfernsee and thus to the Havel River’s extensive network of waterways via the Hasengraben, a short canal where the water is kept dammed up to maintain a high water level for the lake.

This is done to keep the wooden piles on which the Marmorpalais is built from being exposed to the air and drying out.

In summertime the depths can be virtually without oxygen, causing dead ecological zones.