An ecologically unique cirripede, Octolasmis unguisiformis sp. nov (Cirripedeia: Thoracica: Poecilasmatidae), was found to be ectosymbiotic with an ocypodid crab, Macrophthalmus milloti, on tidal flats of the Amami Islands, the Ryukyu Archipelago, Japan. The cirripedes were attached to the ventral side of the crab carapace, around the bases of the first and second ambulatory legs. The cirripedes resemble the chelipeds of male host crabs. Adult cirripedes were found only on female crabs, whereas juvenile cirripedes were present on crabs of both sexes, suggesting that the survival rate of the cirripedes on female crabs was higher than that of those on male crabs. The cheliped-like appearance and female-biased distribution of the cirripedes are discussed from the standpoint of mutualism between the crab and the epizoic cirripede.