Tainai is a city located in Niigata Prefecture, Japan. As of 31 December 2020, the city had an estimated population of 28,495 in 10,821 households, and a population density of 110 persons per km2. The total area of the city was 264.89 square kilometres (102.27 sq mi).

Niigata Prefecture is a prefecture in the Chūbu region of Honshu of Japan. Niigata Prefecture has a population of 2,131,009 and is the fifth-largest prefecture of Japan by geographic area at 12,584.18 km2 (4,858.78 sq mi). Niigata Prefecture borders Toyama Prefecture and Nagano Prefecture to the southwest, Gunma Prefecture to the south, Fukushima Prefecture to the east, and Yamagata Prefecture to the northeast.