Camptandriid crabs collected in the Kumanoe River Estuary, Kyushu, Japan were studied on the basis of morphological characters and molecular analysis. As a result, a new species, Deiratonotus kaoriae, was recognized. These crabs were found mainly in a creek of the sandy tidal flat within the Kumanoe River Estuary. The new species shares a very diagnostic character, the presence of a transverse ridge on the carapace, with D. cristatus (de Man, 1895) and differs markedly from the other congeners that lack this feature. The new species, however, differs from D. cristatus in the absence of harpoon-shaped setae on the subdistal end of the first gonopod and the presence of an extremely reduced second abdominal segment. According to a molecular analysis based on 12S 16S mitochondrial rRNA gene sequences, with Cleistostoma dilatatum (de Haan, 1833) and Camptandrium sexdentatum Stimpson, 1858 as outgroups, Deiratonotus kaoriae is more closely related to D. cristatus than to D. japonicus (Sakai, 1934).