Lake Theiss (Hungarian: Tisza-tó) is the largest artificial lake in Hungary.

As part of the ongoing Theiss River flood control project, the Kisköre Reservoir (Hungarian: Kiskörei víztározó) was built in 1973. Its filling was finished in the 1990s, resulting a 127 sq. km lake. The lake is 27 km in length, with an average depth of 1.3 m and a maximum depth of 17 m; it contains 43 sq. km of small islands.

Following the reservoir's completion, Hungarians began to flock to the site for holidays, since it compared favorably with the crowded and expensive Lake Balaton, the traditional holiday site. As a result, tourist infrastructure has been developed on the reservoir—renamed Lake Theiss—and the government has designated it an official tourism destination.

The lake (or reservoir) has a new local ecology with a diversity of birds, plants, and animals.