Fleesensee, Lake in Mecklenburgische Seenplatte District, Germany.

Fleesensee is a lake in the Mecklenburgische Seenplatte district, about 4.9 km long and up to 4 km wide, with an average depth of 6.1 m. It lies in a chain of lakes between the Müritz and the Plauer See, flows into the Malchower See to the west, and connects to the Kölpinsee via the Fleesenkanal to the east. The Elde flows through the chain from east to west.

The lake lies in a chain of waters between the Müritz and the Plauer See, through which the Elde flows from east to west as part of the Müritz-Elde-Wasserstraße. In 17th-century maps by Ioannes Laurenberg, the whole water body was labelled Calpin lacus (Kölpinsee); the name Fleesensee appears to be of more recent origin. The lakeside resort in Untergöhren opened in 2000.

The lake's name is recent: 17th-century maps label the whole water body Calpin lacus, i.e. Kölpinsee. Today the chain is part of the Müritz-Elde-Wasserstraße (waterway class I), heavily used by excursion boats and watersports, and the Land Fleesensee resort opened in 2000 in Untergöhren, though it does not directly border the lake.

Multiple boat rental stations and launching points around the shoreline provide access for water sports and fishing activities. The best visiting season is May through September when weather is stable and most facilities operate at full capacity.

The lake's present name is not on the oldest maps: in Mecklenburg DUCATAS by Ioannes Laurenberg (17th century) the whole water body is labelled only Calpin lacus, i.e. Kölpinsee. The Fleesensee name therefore appears to be younger, and the Kölpinsee name survives today for the neighbouring lake reached through the Fleesenkanal.