Loboixys ryukyuensis, a new genus and species of the family Notodelphyidae, is described based on females living in the compound ascidian Diplosoma virens (Hartmeyer). The host was collected on the coral reef in Okinawa, Ryukyu Archipelago, Japan. Among notodelphyids, most of which are associated with compound ascidians, the new copepod closely resembles, in morphology, species of Doroixys Kerschner, 1879. In comparison with all known species of Doroixys and two other allied notodelphyid genera, this copepod can be assigned to a new genus on the basis of the following characters: the inflated body is lobed, the antennule is indistinctly 7-segmented, the labrum has four lobes, the fifth leg consists of one inner seta and a 2-segmented outer lobe with one seta, and the setiferous caudal ramus is extremely reduced.