During the project “Research on Deep-sea Fauna and Pollutants off Pacific Coast of Northern Japan", a small collection of cymothoid isopods was obtained at depths ranging from 150 to 908 m. Four species of cymothoid isopods including a new species are reported. Mothocya komatsui sp. nov. is distinguished from its congeners by the elongate body shape and the heavily twisting of the body. Three species, Ceratothoa oxyrrhynchaena Koelbel, 1878, Elthusa sacciger (Richardson, 1909), and Pleopodias diaphus Avdeev, 1975 were fully redescribed. Ceratothoa oxyrrhynchaena and E. sacciger were firstly collected from blackthroat seaperchs Doederleinia berycoides (Hilgendorf) and Kaup's arrowtooth eels Synaphobranchus kaupii Johnson, respectively.