By the survey on the nereidid fauna in small estuaries in creeks in the Ryukyu Islands, southern Japan, two species of Composetia Hartmann-Schröder, 1985, C. kumensis Sato, 2020 (type locality: Kume-jima island, the Ryukyu Islands) and C. tokashikiensis Sato, 2020 (type locality: Tokashiki-jima island, the Ryukyu Islands) have been newly collected from 10 sites on 5 islands, and from 20 sites on 6 islands, respectively. In 26 of a total of 28 sites, one of the two species occurred exclusively, but both of them coexisted in two sites in Oura Bay on Okinawa-jima island. Morphological variations and genetic differentiations among the local populations of the two species were examined. Molecular analyses based on the partial sequences of the mitochondrial cytochrome c oxidase subunit I gene supported that the morphologically similar two species are certainly distinct species. In C. kumensis, marked genetic differentiation was detected among populations in the following three regions in the middle Ryukyu Islands: (1) northern Kume-jima, (2) southern Kume-jima, and (3) Okinawa-jima and Kikai-jima islands. In contrast, no marked genetic differentiation was detected among any populations of C. tokashikiensis in a wide range of the middle and southern Ryukyu Islands. A single specimen of C. tokashikiensis was also collected from an euryhaline area of the lower reaches of Songkhla Lagoon, southern Thailand. A morphological description of this specimen is provided here as a new record of this species from Thailand.