Geiseltalsee, Artificial lake in Saalekreis, Germany

Geiseltalsee is a flooded former lignite opencast mine in the Geiseltal valley, with a surface of 18.4 square kilometres and a maximum depth of 78 metres. It holds about 0.423 cubic kilometres of water, making it Germany's largest artificial lake by area until the Cottbuser Ostsee is fully flooded. Its irregular shoreline of wooded peninsulas and coves lies between Mücheln and Braunsbedra.

Lignite was documented in the Geiseltal from 1698, but large-scale opencast mining began only around 1905/06, with pits such as Elisabeth, Rheinland and Cecilie opening from 1907. Opencast operations ended on 30 June 1993 after 1.4 billion tonnes of lignite had been extracted, and 18 settlements were destroyed with about 12,500 people resettled. Flooding with water from the Saale began on 30 June 2003 and was completed in 2011.

The Geiseltal valley gave its name to the lake and was known for its Eocene fossil beds, exposed by the lignite mine. On about 30 hectares of the northern shore, the Goldener Steiger vineyard was planted, with a Straußwirtschaft wine tavern. The shore slopes are grazed by Rotes Höhenvieh, an endangered cattle breed.

The lake is bordered by Mücheln to the southwest, west and northwest, Braunsbedra to the northeast, east and southeast, and Bad Lauchstädt along a short stretch to the northwest. Three roughly 14-metre observation towers stand near the shore: Seeblick on the Klobikau spoil tip, Leonhardt just north of Neumark, and Pauline near Stöbnitz, 1 km north of the marina there. The water level is held at 98.05 m above sea level by the Frankleben outflow structure.

The lake bed is the former opencast mine of the Geiseltal, where the fossil record made the valley famous; the pit was about 80 metres deep and covered some 2,600 hectares before flooding. Water from the Saale is still added because the fractured mining subsoil loses water through seepage.