Dayan Lake (Mongolian: Даян нуур, Chinese: 达彥湖) is a lake located the district of Sagsai, in the Bayan-Ölgii Province of western Mongolia.

It is recognized by BirdLife International as an Important Bird Area since 2009.

Dayan Lake remains frozen from October to June.

Six species nest around the lakes that BirdLife has identified as key.

The other three are considered to be of least concerns; they include: northern lapwing (Vanellus vanellus), lesser kestrel (Falco naumanni) and bar-headed goose (Anser indicus).