Lake Lungern (German: Lungerersee, also spelled Lungernsee or Lungerensee) is a natural lake in Obwalden , Switzerland which is named after the town Lungern on its shore. The lake is drained by the Sarner Aa river, which flows through the Sarnersee and into Lake Lucerne .

The lake was originally much larger, and covered a large part of the valley it is situated in. Starting in 1836, its level was lowered by 36 metres (118 ft) through an artificial drainage tunnel with a length of 380 metres (1,250 ft). It is used as a reservoir.