A new species, Dimorphostylis namhaedoensis (Cumacea, Diastylidae) is described on the basis of the specimens collected from southern shallow coast of Korean peninsula. The new species resembles four other species, D. asiatica, D. valida, D. hisuta and D. acroplicata, in having three apical spines on the terminal margin of the male telson and three pairs of the oblique ridges on the carapace, but are obviously distinguished from them in that the pattern of the ridges on the carapace is different and the third article of the antennular peduncle is very long in female. Furthermore, the dorsal carina of the female carapace has a row of forward spines and among the three apical spines of male telson, the median spine is much smaller than the lateral ones.