Lake Suwa is a lake in the central part of Nagano Prefecture, Japan. It ranks 24th in Japan in surface area.
Lake Suwa is the site of o-miwatari, an interesting natural phenomenon. The lake has a natural hot spring under the surface, so that when the top freezes in the winter, the lower waters are still warm and circulating. This results in pressure ridges forming in the ice, reaching heights of 30 cm or more. Local belief holds that the pressure ridges are formed by the gods crossing the lake, travelling between the various buildings of Suwa Taisha.