Heiliger See, Lake in Nördliche Vorstädte, Germany.

Heiliger See is a lake in Potsdam's Nördliche Vorstädte, bordered by the Berliner Vorstadt and the Neuer Garten. It measures 1.33 km north to south and 400 m at its widest, with an average width of 300 m. Fed by groundwater, it has no surface inflow; the Hasengraben canal at its north end links it to the Jungfernsee. The Gotische Bibliothek stands on the south shore.

The lake lies in a glacial tunnel valley and forms a chain with the Sacrower See and Groß Glienicker See. The Hasengraben canal at its north end was dammed to keep the water level high, protecting the wooden foundation piles of the Marmorpalais on the west shore from drying out. Since 2017 the lake has lost considerable water in summer; by August 2022 the Hasengraben was almost dry.

The eastern shore's Manger- and Seestraße are lined with villas, many under monument protection: Landhaus Rubinski at Seestraße 45, Landhaus Prölls at 41–42, and Villa Metz at 35–37, known from 1999 to 2017 as "Villa Wunderkind" after fashion designer Wolfgang Joop.

The lake has no surface inflow; it is fed by groundwater and drains north through the dammed Hasengraben toward the Jungfernsee. In summer the deep water is low in oxygen, and since 2017 the lake has lost considerable water, with the Hasengraben nearly dry in August 2022. Unofficial bathing spots and sunbathing meadows lie on the north shore; the Gotische Bibliothek stands at the southern tip by the park's south entrance.

The water level is carefully managed to protect the wooden foundation piles of the nearby Marmorpalais from damage. This technical control is necessary because improper water levels could endanger the historic structures.