Lake Tazawa, Caldera lake in Semboku, Akita Prefecture, Japan

Lake Tazawa is a crater lake in Semboku that covers 25.9 square kilometers in northern Akita Prefecture. Sapphire-blue water reflects the surrounding mountain forests, creating a natural frame of wooded hills along the shoreline.

Lake Tazawa was long known as Tazawa-no-kata or Tatsuko-ga-kata before the name Tazawa-ko took hold in the Meiji period. In 1940 a channel diverted strongly acidic water from the Tamagawa River into the lake for hydropower and irrigation, killing most fish, including the endemic kunimasu. Liming began in 1972 and a neutralization plant opened in 1991.

The lake's name was fixed only in the Meiji period; older records call it Tazawa-no-kata or Tatsuko-kata, and one explanation traces "Tazawa" to the Ainu tapukopu, "hill with a raised circular top". Four statues of the maiden Tatsuko stand around the shore, among them Yasutake Funakoshi's bronze Tatsuko, unveiled on 12 May 1968, and a Tatsuko-hime figure in the Goza-no-ishi Shrine grounds on the north bank.

Buses from JR Tazawako Station bring visitors to several access points along the 20-kilometer shoreline. The paths around the shore are suitable for walking, and in summer you can swim or rent pedal boats.

The kunimasu, a fish found only in Lake Tazawa, was declared extinct after the 1940 inflow of acidic Tamagawa water. In 2010 a population was confirmed in Lake Saiko, Yamanashi Prefecture, where eggs had been stocked, and the two lakes became sister lakes in November 2011. A lakeside museum opened on 1 July 2017 to display kunimasu borrowed from Yamanashi.