Lake Sempach (German: Sempachersee) is a lake in the canton of Lucerne, Switzerland. Its area is about 14.5 km (5.6 sq mi) and its maximum depth is 87 m (285 ft).

A 50-metre-large (160 ft) island named Gamma Insel is located east of Sursee.

In the 17th century the lake counted with five ferries, that brought goods to the weekly market in Sursee.

Many inhabitants of local areas swim, boat, paddle, and fish in Lake Sempach, due to its ease of accessibility.