The Sannōkai Dam (山王海ダム, Sannōkai damu) is a dam on the Toyosawa River, a tributary of the Kitakami River system, located in the town of Shiwa, Iwate Prefecture in the Tōhoku region of the island of Honshū, Japan.

[1] The need for storage reservoirs in the Kitakami River valley for irrigation purposes was recognized by the Meiji government at the start of the 20th century, due to repeated crop failures and conflicts between various communities over water rights, dating even from the early Edo period.

The river also suffered from severe environmental problems with acidic runoff from upstream mining operations.

Plans for a series of dams was initiated in 1941 by the Ministry of Agriculture and Forestry, but all work was halted during World War II.

However, due to aging and concerns over safety, it was completely rebuilt starting in 1977 as a rock-fill dam, with its height raised to 61.5 meters.