A new species of the Cletopsyllidae belonging to the genus Isocletopsyllus Huys and Lee, 1999 is described from the materials collected from subtidal sandy bottoms at Jeju Island and Ulleung Island, Korea. So far, only two species of the genus Isocletopsyllus, viz. I. tertius (Por, 1964) from Israel and I. quartus (Soyer, 1966) from Banyuls-sur-Mer (France) are recorded, both from the Mediterranean Ocean. The new species can be clearly distinguished from its congeners by the combination of the following characters: (1) female antennule with a large process on the outer margin of first segment, (2) caudal rami with a bulbous proximal inner expansion, and shorter than those of other species (about four times long as wide in the new species), (3) second segment of P1 exopod with a short rod-like inner seta, (4) sexual dimorphism presented in the second endopodal segment of male P2, not in exopod of P4, and (6) female P5 with a very long exopod with very short second outer seta. A table comparing the modifications of swimming legs in the family Cletopsyllidae Huys and Lee, 1999 is presented. This is the first record of this family from Korea.