A new species of swimming crab of the genus Catoptrus A. Milne-Edwards, 1870 (Portunidae) is described from Kagoshima, southern Japan. Catoptrus lavicolus n. sp. was collected from deep inside large lava rocks at shallow subtidal bed of Sakurajima, an active volcano. The new species differs from all seven congeners by a combination of the characters of the carapace, chelipeds and ambulatory legs. Intersex specimens of C. lavicolus n. sp. were observed in this study, while no male specimen has been collected.