Lake Inawashiro, Tectonic lake in Fukushima Prefecture, Japan.

This body of water covers 104 square kilometers at an altitude of 514 meters, with depths reaching 94 meters in places across central Fukushima Prefecture. The volcanic cone of Mount Bandai rises directly from the northern shore, while smaller mountains frame the other sides.

The basin began forming after the Quaternary period through the Kawashiro Fault on the east and the Aizu Basin eastern margin fault on the west. Pyroclastic flows from Mount Bandai, the Okunoshima deposit about 90,000 years ago and the Zunashi deposit about 42,000 years ago, dammed the drainage, raising the water level. Erosion by the Nihashi River then cut the surface down to today's 514 metres.

The lake is also called Tenkyōko, "Heavenly Mirror Lake", for the clarity of its water. The name Inawashiro itself is not tied to a castle; local legend instead explains the small island of Okina-jima: a poor woman named Okina gave the monk Kōbō Daishi water to drink, and when Mount Bandai erupted and 52 villages sank, only her house remained above the water as an island. Sightseeing boats from Nagahama still circle that island.

Three channels bring water from here to Koriyama city, while the surrounding region uses several hydroelectric plants. Main roads run along the western, southern, and eastern sides with several public access points.

The lake drains to both the Sea of Japan and the Pacific: the Nihonbashi River leads to the Sea of Japan via the Agano system, while the Asaka Canal crosses the watershed by tunnel to the Abukuma system. In the 1950s its water powered plants that supplied Tokyo, earning it the name "Lake of Electricity".