In this study, a new species, $Dimorphostylis$ $breviplicata$ n. sp. is described and illustrated on the basis of specimens collected from the shallow Korean waters. This new species is similar to $Dimorphostylis$ $brevicaudata$ (Zimmer, 1903), $Dimorphostylis$ $acroplicata$ Harada, 1960 and $Dimorphostylis$ $valida$ Gam$hat{o}$, 1962 in that the carapace has three pairs of oblique ridges on the surface. However, the new species is distinguished by the combination of the following features: the middle oblique ridge is short and not joined to the other oblique ridges; the transverse ridge is absent on the frontal lobe; the dorso-median process on the pereonite 5 is only barely visible; the male telson has no short stout seta on the both lateral margins; the length ratio of the first to remaining articles combined on the male uropod is 1 : 0.7. A key to the Korean $Dimorphostylis$ species is provided. Also, $Diastylis$ $alaskensis$ Calman, 1912 is redescribed as a new recorded species of the Korean fauna.