Tetraclita squamosa and Tetraclita japonica have recently been separated into two species using allozyme and DNA analysis. The morphologies of the two species, however, have not been described fully, resulting in confusion over diagnostic characters and identification. Tetraclita squamosa has greenish parietes and the tergo-scutal flaps are black with two pairs of white spots. Tetraclita japonica has purplish-grey parietes and the tergo-scutal flaps are not patterned. These characters are good for in-situ species identification. The third cirrus of T. japonica possess an additional bidentate-type seta, which is absent from T. squamosa. Results of discriminant function analysis indicate the shape of the parietes and the scutum geometry of T. squamosa and T. japonica show intraspecific variation between sites, indicating they are not diagnostic characters for separating the two species. The basi-scutal angle of the tergum, however, is significantly different between the two species and is useful in distinguishing the species. By means of these specific criteria, the two species can be identified with confidence on the shore and in the laboratory.