The genus Callophyllis is recorded as six separate species with imprecise species delimitation in Korea. To elucidate the species boundaries of Korean Callophyllis, we performed morphological observations and molecular analyses, and included three Japanese Callophyllis species from the type locality. From the results of molecular analyses using plastid rbcL and mitochondrial COI-5P genes, we confirmed ten Callophyllis species, including five cryptic ones: C. adhaerens, C. adnata, C. crispata, and C. japonica from Korea and Japan; C. hayamensis as an unrecorded species from Korea; C. cartilaginea, C. mollitia, C. repens, C. serratifolia, and C. undulata as new species from Korea. There were no Korean specimens that matched C. adnata or C. crispata from Japan, except Korean C. japonica, which formed a genetic group with the Japanese species. We obtained the interspecific divergences among the five cryptic species as 0.6-4.5% in rbcL and 2.8-8.4% in COI-5P. We recognized that the species diversity of Callophyllis has been underestimated from the northwestern Pacific region. The species boundary of Callophyllis from Korea and Japan will be a cornerstone to revealing the phylogenetic affinity of the genus distributed in both hemispheres of the western Pacific.