Intertidal mudflat crabs of the genus Metaplax H. Milne Edwards, 1852 ( Crustacea : Brachyura : Varunidae ) from China, Taiwan, and northern Vietnam are taxonomically revised by morphological and molecular evidence. These crabs show sexual dimorphism and morphological variation of a considerable range in the infraorbital ridge, one of the primary features previously used for species identification. In this study, four species were identified from the region: M. elegans De Man, 1888; M. longipes Stimpson, 1858; M. sheni Gordon, 1930; and M. tredecim Tweedie, 1950. Based on the results of the morphological examination, and as confirmed by molecular evidence from mitochondrial cytochrome oxidase subunit I (COI), taxonomic confusion surrounding M. longipes was resolved, and M. takahasii Sakai, 1939, is considered a junior synonym of M. longipes . The geographical distribution of Metaplax longipes extends along the shores of China, north to Jiangsu, whereas the Southeast Asian M. tredecim was newly recorded from northern Vietnam and Hong Kong.