Ammersee (English: Lake Ammer) is a Zungenbecken lake in Upper Bavaria, Germany, southwest of Munich between the towns of Herrsching and Dießen am Ammersee. With a surface area of approximately 47 square kilometres (18 sq mi), it is the sixth largest lake in Germany. The lake is at an elevation of 520 metres (1,710 ft), and has a maximum depth of 81 metres (266 ft). Like other Bavarian lakes, Ammersee developed as a result of the ice age glaciers melting. Ammersee is fed by the River Ammer, which flows as the Amper out of the lake. Like neighbouring Lake Starnberg, which is similar in size and shape, it is a popular location for water sports. The lake's water generally is of very good quality since a circular sewerage system has been introduced in the 1960s collecting all waste water from around the lake and transporting it to a treatment plant below the lake's outlet at Eching.
Ammersee and the Amper are part of the ancient Celtic amber trading route leading to the Brenner Pass.
The word Ammer is a 13th-century form of Amper, the Celtic *ambra, deriving from the Indo-European *ombh-, *mbh- "wet, Water".
Passenger services have operated on the lake since 1879. Today they are operated by the Bayerische Seen schifffahrt company, using a mixture of historic paddle steamers and motor ships