A poorly known sculpin, Atopocottus tribranchius Bolin, 1936 (Cottidae), was redescribed on the basis of 56 specimens from several localities in Japan; off Yamaguchi, Kyoto and Niigata Prefectures on the Japan Sea coast, and Kochi Prefecture on the Pacific coast, and in the northern East China Sea. The specimens from Yamaguchi, Kochi and the East China Sea represent the first specimen-based records for the species from those regions. Minute organs on the trunk surface, previously regarded as lateral-line pores, are most likely epidermal pits containing free neuromasts, and lateral-line pores are restricted to a row of embedded lateral-line scales on the anterodorsal portion of the body. Diagnostic characters of the monotypic genus, Atopocottus Bolin, 1936, are here reviewed on the basis of external and selected osteological characters, described for the first time. The genus is more similar to the Southern Hemisphere (Indonesia, Australia and New Zealand) genus Antipodocottus Bolin, 1952 than to Stlengis Jordan and Starks, 1904, known only from Japan, in having poorly developed cephalic sensory canals with few pores and the lateral-line scales in a simple canal without dorsal and ventral branches. However, Atopocottus differs from Antipodocottus in having fewer lateral-line scales (3–5 vs. 5–33 in the latter); the uppermost preopercular spine with 1–6 dorsal branches, their length and width becoming shorter and narrower, respectively, toward the spine tip, some of them with two spinous points (vs. 2–5 simple, similarly sized dorsal branches); and lacking a supraocular tentacle (vs. usually present). A more comprehensive examination of the two genera is necessary to determine their monophyly.