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The Asian soldier crab Mictyris brevidactylus Stimpson, 1858, was re-examined genetically and morphologically from specimens collected across its range. It was found to consist of two cryptic sister species, with crabs from the Ryukyu Islands representing a new species, M. guinotae. Mictyris guinotae is smaller at maturity than M. brevidactylus, has plain blue colouring without red banding on the legs, and has a differently shaped apex of its male first gonopod. The two species also differ genetically, with 16S rRNA and COI showing a mean divergence (K2P distance) of 1.08% and 4.39%, respectively, representing 2.0 and 3.5 times more divergence than present intraspecifically. Results provide further evidence that some elements of the marine fauna of the Ryukyu Islands have had a separate evolutionary history from continental Asia as well as Taiwan.