The Hengsteysee (Lake Hengstey) is a reservoir on the Ruhr river between the cities of Hagen, Dortmund and Herdecke, North Rhine-Westphalia, Germany.

It begins near the point where the Lenne flows into the Ruhr, and ends with the weir and hydroelectric plant of Hengsteysee.

The Klusenberg, a hill that is part of the Ardey range, is located just north of the Hengsteysee.

There is also a pumped-storage plant on this reservoir (called the Koepchenwerk after Arthur Koepchen), along with a 4.5 MWh grid services battery repurposed from electric cars.

The health department has also expressly warned against this since September 2008, when several people had to seek medical treatment after coming into contact with the lake water.