We found an endangered sedge, Carex kujuzana Ohwi on the Sotochichibu mountains of Saitama Prefecture, eastern Japan. This is the first record of the species in Saitama Prefecture. In the study area, C. kujuzana occurred on ridges and mountain tops at elevations of 560–670 m in the distribution area. The habitats were sparse woods, semi-natural grasslands, and deciduous broad-leaved secondary forests, which were consistent with other prefectures where it is found. According to aerial photo interpretation, these sites were kept as grasslands, shrublands, and sparse woods throughout the 1940s, 1970s, and 2000–2010s, and some historical documents state that the area has been used as secondary grasslands since the Edo period. Therefore, the habitats of this plant are considered to have been under human management for a long time. In Saitama Prefecture, the C. kujuzana distribution is confined to small areas, and the habitats are rapidly deteriorating due to plant succession.