Koshigaya's 2026 population is now estimated at 391,861. In 1995, the population of Koshigaya was 298,253. Koshigaya has grown by 8,859 in the last year, which represents a 2.31% annual change. These population values and projections come from Japan city census series 1995 to 2005 via CityPopulation fallback access, Japan 2010 Census municipal population Table 1 (e-Stat official CSV), Japan 2015 Census municipal population Table 1 (e-Stat official CSV), Japan 2020 Census municipal population Table 1-3 (e-Stat official download) and World Population Review Internal Projections .