Brackish-water calanoid copepods, defined as those predominantly inhabiting oligo- and mesohaline wa- ters, were investigated at 45 rivers in western Japan during 2006-2009 to review their faunas and, for comparison, 4 rivers of Korea in 2006. Eight species (Acartia ohtsukai, A. sinjiensis, A. tsuensis, Pseudodiaptomus inopinus, P. ishigakiensis, Sinocalanus sinensis, S. tenellus and Tortanus derjugini) were identified as brackish-water calanoid copepods from Japan. Among them, P. inopinus was most often collected; it occurred in 38 rivers. Acartia tsuensis and A. sinjiensis were also frequent species in Japan. Pseudodiaptomus ishigakiensis discovered from Kyushu Island is new to the mainland of Japan. From the rivers of Korea six species were identified (A. ohtsukai, A. sp., P. inopinus, P. sp., S. tenellus and T. dextrilobatus). Based on the present and previous studies, a total of 13 brackish-water calanoid copepods have been recorded from Japan and Korea, of which eight species, including three continental relicts (A. ohtsukai, S. sinensis and T. derjugini) in Kyushu Island, are common to both regions. Among them, only two species (P. inopinus and S. tenellus) are distributed widely on both Japanese and Korean coasts of the Tsushima (Korea) Strait. This indicates that the faunas of brackish-water calanoid copepods are very different between Japan and Korea despite the short geographical distance (about 170 km) and that the Tsushima Strait has played the role of an effective barrier for their dispersion.