Engstlensee, reservoir in Innertkirchen, Bernese Oberland, Switzerland

Engstlensee is a mountain lake in the canton of Bern situated about 1851 meters above sea level in a narrow valley. The lake covers about 0.44 square kilometers and reaches almost 50 meters in depth at its deepest point, allowing it to hold over 10 million cubic meters of water.

The Oberhasli power plants (KWO) use Engstlensee as a reservoir, drawing about 1 million m³ of its 10.7 million m³ for electricity. A 600 m intake tunnel on the right bank feeds water into the Gental, then through a shaft into the Gadmertal, where it drives the Fuhren power station. The lake can be lowered five metres in winter.

An inn on the Engstlensee shore is the setting for a decisive meeting between the main characters of H. G. Wells' 1913 novel The Passionate Friends. The lake lies on the Engstlenalp, and the surrounding Trüebsee, Tannensee and Melchsee are often walked together with it on routes from Engelberg down to Melchsee-Frutt.

Engstlensee lies at about 1851 m at the head of the Gentel valley, below the Joch Pass (2207 m) and the Titlis. It is reached on foot or by cable car from Engelberg; a common route continues past the Trüebsee, Tannensee and Melchsee lakes down to Melchsee-Frutt. The inlet of the 600 m diversion tunnel is on the right bank.

The 600-metre intake tunnel starts on the lake's right bank and empties into the Gental water about 250 metres below the natural outflow; after 480 more metres the water is captured and led through a tunnel into the neighbouring Gadmertal. There it drives the Fuhren power station, and can later be used at Hopflauenen and Innertkirchen 2 or pumped up to the Räterichsboden reservoir by the Handeck 3 pump turbine.