The Flakensee is an inland water located east of Berlin in the Oder-Spree district, Brandenburg. It is part of the federal waterway in Rüdersdorfer Gewässer with waterway class III, which mainly transports cement from the Rüdersdorf cement works to Berlin and Poland. The Berlin Water and Shipping Authority is responsible for the federal waterway.
The Flakensee has its inflow from the north from the Kalksee over the Kalkfließ and in the southeast over the Löcknitz, another federal inland waterway. It drains to the southwest over the flake flow through Erkner to the Dämeritzsee. At the south end of the limestone flow is the Woltersdorf lock with a head of 2.10 meters. The municipality of Woltersdorf is in the north of Flakensee, and the city of Erkner in the south.
In addition to shipping, the 67 hectare and a maximum of 8 meters deep body of water is mainly used for water sports and as a swimming lake.