A new species of semi-terrestrial sesarmid crab of the genus Geosesarma De Man, 1892, is described from central Taiwan. Geosesarma mirum sp. nov. is distinct in possessing a strong transverse crest on the inner surface of the male chela and a diagnostic male first gonopod which is relatively long and stout, with the distal chitinous part broad and spatuliform. Like most Geosesarma species, G. mirum sp. nov. has large eggs and direct development, contrasting with the only other species known from Taiwan, G. hednon Ng, Liu and Schubart, 2004, which has small eggs and planktotrophic larvae.