Greifensee (Swiss Standard German pronunciation: [ˈɡraɪfn̩ˌseː]) is a natural lake in the canton of Zürich in Switzerland .

Greifensee is located 11 km (6.8 mi) to the east of the city of Zürich, separated by the Pfannenstiel from Lake Zurich . As the second largest lake in the canton of Zürich (Lake Zurich being the largest), it is about 6 km (3.7 mi) long and 1.6 km (0.99 mi) at the widest point, with a maximum depth of 32 m (105 ft). The Aabach (Greifensee) (or just Aa) is the main supplying river, while its outlet is the Glatt. On its southeastern end the Mönchaltorfer Aa (or just Aa) enters the Greifensee.

The lakeside is under UNESCO protection, and buildings are not allowed, resulting in reed bed and a rich fauna and flora: Around 400 plant species in the lake and 19 species in its tributaries.