Tomioka (富岡市 Tomioka-shi) is a city located in southwestern Gunma Prefecture in the northern Kantō region of Japan. As of February 2015, the city had an estimated population of 49,873 and a population density of 406 persons per km². Its total area was 122.85 km². It is the location of the Tomioka Silk Mill, a UNESCO World Heritage Site.