A new genus, Ryukyum, is established for the terrestrial potamid crab from Ishigaki and Iriomote Islands, the southern Ryukyus, Japan, Nanhaipotamon yaeyamense Minei, 1973. The new genus differs from Nanhaipotamon s. str. in the structure of the sternum, third maxilliped, male abdomen, and male first and second gonopods.