Mirograpsus asper is described as a new genus and new species in the subfamily Ilyograpsinae Števčić, 2005, of the family Macrophthalmidae Dana, 1851, on the basis of a single male specimen from Ishigaki Island, Ryukyu Islands, Japan. The new genus most closely resembles the monotypic Apograpsus Komai Wada, 2008 (type species, Ilyograpsus paantu Naruse Kishino, 2006), sharing a similar carapace shape, but is differentiated chiefly by the fusion of the male pleomeres 4-6 and the unarmed meri of the ambulatory legs. The functional fusion of the male pleomeres 4-6 is unique in the Macrophthalmidae.
Keywords: Crustacea, Apograpsus paantu, Ilyograpsinae, Ishigaki Island, Mirograpsus asper.