Lake Dai, also known by its Chinese name Daihai, is a lake in Liangcheng County, Ulanqab Prefecture, Inner Mongolia, China.
Lake Dai is a round lake with an area of about , measuring about from east to west and about from north to south. It has an average depth of about and a maximal depth of .
Under the Qin, Huangqi and Dai lakes formed the northern boundaries of Yanmen Commandery, marking part of the northern frontier of the Chinese empire.