We report somatic chromosome numbers 2n = 12 for three Carex sect. Siderostictae Franch. ex Ohwi (Cyperaceae) from Korean populations: Carex ciliatomarginata Nakai, C. okamotoi Ohwi, and C. siderosticta Hance. This study is the first chromosome number report for the species C. ciliatomarginata from Korean populations. As found in other Carex species, all the chromosomes examined in the section exhibit nonlocalized centromere (polycentric or holocentric) and large (more than ca. 1 μm long) chromosomes. Considering the basal phylogenetic position of the section in tribe Cariceae Pax, small numbers of large chromosomes have been hypothesized as primitive characters in Cariceae, and our observation supports the hypothesis. Further investigations of chromosomes in Carex are needed for a better understanding of species richness in the genus.