Lake Kizaki, Tectonic lake in Ōmachi, Japan.

Lake Kizaki is a tectonic lake in Ōmachi with a long, narrow shape and steep sides that define its basin. The water body is surrounded by forested hills, creating a well-defined landscape feature.

Lake Kizaki lies in a graben of the Itoigawa-Shizuoka Tectonic Line, the fault zone that runs through the Nishina Three Lakes. Its basin was dammed when gravel from the Kashima River fan piled up against displaced fault blocks, and a step of about 10 metres on the lake floor marks the Matsumoto Basin eastern-margin fault. The lake is the southernmost and lowest of the three.

The lake was featured as a filming location for several Japanese animated series, making it recognizable to fans of these productions. Visitors can identify the spots where scenes from these shows were shot.

Three JR Ōito Line stations serve the lake: Shinano-Kizaki, Inao and Uminokuchi. The lakeside park at the southern end, rebuilt by May 2007, has a walking path and serves as the main gateway. The lake surface sits at 764 m, and summer surface water reaches 28 °C.

Lake Kizaki is home to the Kizaki trout, a fish found nowhere else: a natural cross between yamame and amago or biwamasu, it can reach 60 cm. Overfishing made it rare until 1985, when the fisheries cooperative and Nagano Prefecture began breeding and releasing fry.