Species of the bopyrid isopod genus Rhopalione Pérez, 1920, are parasites of Indo-West Pacific pinnotherid crabs. Unlike other bopyrid parasites of brachyurans that occupy the branchial chambers, however, species of Rhopalione (subfamily Pseudioninae) infest the abdominal cavity. Prior to the present study, four species of Rhopalione were recognized: R. atrinicolae Page, 1985 (New Zealand), R. incerta (Bonnier, 1900) (Madagascar), R. sinensis Markham, 1990 (East Asia), and R. uromyzon Pérez, 1920 (Persian Gulf). A fifth species of Rhopalione, from Perhentian Besar, Malaysia, is described herein, parasitic on the pinnotherid crab Serenotheres besutensis (Serène, 1967). Keys are provided to females and males of the species in the genus.