Tomakomai is a city and port in Iburi Subprefecture, Hokkaido, Japan. It is the largest city in the Iburi Subprefecture, and the fourth largest city in Hokkaido. As of 31 July 2023, it had an estimated population of 167,372, with 83,836 households, and a population density of 298 persons per km². The total area is 561.57 km2 (216.82 sq mi).

Hokkaido is the second-largest island of Japan and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by the undersea railway Seikan Tunnel.