Three species of the genus Prosopistoma Latreille, 1833 (Prosopistomatidae) are currently reported from Thailand. A new species, Prosopistomacarinatumsp. n., is described here based on specimens from western and southern Thailand. The new species can be easily distinguished from the other members of Prosopistoma by the following combination of characteristics: (i) the presence of two ridged longitudinal lines on each side of its carapace, (ii) antenna 7-segmented, (iii) a strongly convex carapace and (iv) nine pectinate setae on the ventral margin of the fore tibiae. A comparison between the key characteristics of P.carinatumsp. n. and the known Thai species is provided. Results of analysis of the mitochondrial cytochrome oxidase I (COI) gene (658 bp) of three species, as well as the distribution of the Thai species, are also discussed.