Asurvey focusing on species belonging to the diatom genus Pseudo-nitzschia was conducted in the western North Pacific (the northwestern Sea of Japan and the Sea of Okhotsk). Light and electron microscopic examination of 314 phytoplankton field samples collected from 1995 to 2006 revealed the presence of 11 Pseudo-nitzschia species: P. americana, P. cf. caciantha, P. calliantha, P. delicatissima/arenysensis, P. fraudulenta, P. cf. heimii, P. multiseries, P. multistriata, P. obtusa, P. pungens and P. seriata. Detailed morphological descriptions and illustrations as well as distributional data are provided for all 11 Pseudo-nitzschia taxa. The study presents a taxonomical baseline investigation of Pseudo-nitzschia from the western North Pacific and provides distributional data from an area otherwise not thoroughly examined earlier. Morphological deviation from the current description of P. cf. heimii, P. obtusa and P. caciantha is presented. It is established that P. calliantha, P. multistriata, P. multiseries and P. pungens occur in bloom proportions and are widely distributed in the Sea of Japan and the Sea of Okhotsk. In the literature, P. seriata is characterized as being restricted to the northern hemisphere where it occurs in the North Atlantic Ocean exclusively. The current study, however, documents that P. seriata is found in the North Pacific and hence is widespread in the northern hemisphere. One species, P. cf. caciantha, is a new record for the western North Pacific, whereas two others, P. cf. heimii and P. multistriata, are new records for the Sea of Okhotsk. The geographical distribution of P. cf. caciantha, P. multistriata and P. obtusa in the North Pacific is expanded by the current study.