The reduviid genus Ploiaria Scopoli, 1786 is reported from Japan for the first time, and two new species P. ryukyuana and P. takaii are herein described. P. ryukyuana can be distinguished from its allied species P. thetis Wygodzinsky and Usinger by the one basal seta distantly placed from other setae in the anteroventral series of setae of forefemur, etc. P. takaii can be separable from its allied species P. ryukyuana sp. nov. by the midfemur with a wide pale marking apically and so on. They both inhabit grasslands of silver and blady grasses in the Ryukyus.