Rumoi, city, northwestern Hokkaido, Japan. It lies at the mouth of the Rumoi River, facing the Sea of Japan. The city’s natural port was a fishing centre until the disappearance of herring along its coast after World War II, although cod and Alaska pollack continued to be caught on a small scale. Rumoi is now engaged in the shipping of timber and coal from its hinterland. It is also the seat of the Rumoi subprefectural government. Pop. (2005) 26,826; (2010) 24,457.