Two new species Composetia kumensis and C. tokashikiensis (Nereididae) are described based on specimens collected from subtropical small estuaries in the Kume-jima and Tokashiki-jima islands in the middle Ryukyu Islands, southern Japan, respectively. Both species have the following diagnostic characteristics of Composetia Hartmann-Schröder, 1985: (1) having conical paragnaths in the maxillary ring of the proboscis and lacking paragnaths or papillae in the oral ring, (2) prostomium with entire anterior margin, (3) the absence of falcigers among notochaetae, and (4) the absence of simple chaetae among upper neurochaetae. These new species share the following diagnostic characters: (1) presence of notoacicula on chaetigers 1 and 2, (2) absence of notopodial prechaetal lobe throughout body, (3) presence of neuropodial postchaetal lobe only in anterior body, (4) neuropodial falcigers all heterogomph, and (5) oral ring greatly enlarged in full-everted proboscis. However, C. tokashikiensis sp. nov. is distinguishable from C. kumensis sp. nov. by the presence of heterogomph spinigers among the upper neurochaetae around chaetiger 5. A list of all of 34 species currently belonging to Composetia and a key to Japanese species of Composetia are also provided.