Two marine isopods, Pentias hayi Richardson, 1904 and P. thompsoni Collinge, 1916 were verified as unrecorded species in South Korea. The genus Pentias is reported for the first time in South Korea. These two isopods were collected from Heuksan-do Island and Ulleung-do Island from South Korea, respectively. We provide descriptions of the diagnostic characteristics, illustrations of two species and their partial sequences of the mitochondrial cytochrome c oxidase subunit 1 (CO1) for molecular characteristics.