A species delimitation study of Korean CafiusStephens, 1829 using molecular characters (COI and 28S) is presented. Six coastal species are recognized, one of which is described as Cafius calculosus Lee & Ahn sp. nov. in the intertidal zone of the southern part of the Korean peninsula based on morphological and molecular characters. The new species is closely related and very similar to C. mimulus (Sharp, 1874). Interspecific genetic divergence of COI using uncorrected p-distance ranged from 10.77% to 18.78%, while intraspecific divergence ranged from 0.00% to 2.51% among six Korean Cafius species. All six species were each strongly supported as a single lineage using COI and 28S on both neighbor-joining and parsimony trees. A description, habitus photograph and diagnostic characters of the new species and a key to the species of Korean Cafius are provided to facilitate identification.