Commensal leucothoid amphipods have been collected from the branchial chambers of their ascidian hosts throughout the Ryukyu Archipelago, Japan. Seven new species are described in two genera with valuable location data and host records. An identification key to ascidian-dwelling Leucothoidae of the Ryukyu Archipelago is provided.
Keywords: Japan; Leucothoe amamiensis; Leucothoe elegans; Leucothoe nathani; Leucothoe obuchii; Leucothoe trulla; Leucothoe vulgaris; Leucothoidae; Okinawa; Paranamixis thomasi; Ryukyu; commensal; new species.