Four sand-dwelling species of the marine dinoflagellates genus Sinophysis, including one new species, have been examined from intertidal and subtidal sand, from the Sea of Japan. Morphological features of this species were observed by light and scanning electron microscopy. Sinophysis minima sp. nov. is flattened laterally and is 17.5-35.0 µm in length, and 15.0 - 27.5 µm in depth, with a length/depth ratio of 1.1-1.4, an epitheca depth of 5.0-7.5 µm, and a sulcus length of about three-quarters the hypotheca length. Sinophysis ebriola (Herdman) Balech, Sinophysis grandis Hoppenrath and Sinophysis stenosoma Hoppenrath were recorded for the first time in the seas of Russia. Sinophysis stenosoma is the most common species in the Sea of Japan. All species usually occurred together in the investigated region. Additional information on the known species is provided.