Asiacaris n. gen. is described to accommodate Asiacaris dispar n. sp., collected in the interstitial of the banks of a river on Pha-ngan Island, in the Gulf of Thailand. Asiacaris belongs to Parastenocarididae Chappuis, 1940, based on the following characters: leg 3 sexually dimorphic, modified in the male into a grasping organ; presence of sexual dimorphism on the endopod of leg 4; presence of a dorsal integumental window on cephalothorax and dorso-elliptical integumental windows on tergites of urosomites 2 to 5 in male and on genital double-somite and urosomites 4 and 5 in female; antennule 8-segmented, with an inner process on the 7th segment, first leg with 2-segmented endopod and unarmed exopod-2, and mouthparts armature and segmentation. The apomorphic characters of Asiacaris are represented by the sexual dimorphism of P2 (stronger in male than in female), the fusion of the P5 to the intercoxal sclerite and to the somite and, mostly, by the overall transformation of P4 in the male, which is the longest and strongest leg, a feature never recorded before in any male of freshwater or marine free-living harpacticoids: the endopod is missing, the exopodal segments are all enlarged, and the apical seta is transformed into a hook longer than the last segment of the exopod. The P4 project laterally, creating not only a characteristic habitus, but also possibly a locomotion pattern different from that of all other Parastenocarididae. This modified P4 could be used to grasp the female during coupling, because it forms a second set of pincers, longer and stronger than the one created by the P3.