Kumagaya, city, northwest-central Saitama ken (prefecture), east-central Honshu, Japan. It lies on the Ara River, at the western edge of the Kantō Plain.
The city was named for the 12th-century warrior Kumagai Naozane. It was a post town and silk market during the Edo (Tokugawa) period (1603–1867) and marked the terminus of transport on the Ara River. The central part of Kumagaya was destroyed by Allied bombing during World War II, but the city has since grown to be the commercial, administrative, and transport centre of northern Saitama prefecture. The silk-reeling industry is supplemented by heavy industry, introduced since 1961. Pop. (2000) 156,216; (2010) 203,180.