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Chaetoceros atlanticusvar. koreanus, a new variety of C. atlanticus, was collected from the East Sea of Korea on 20 August 2013. Its morphological characteristics were examined from wild material, and the new variety was compared with three closely related taxa, C. atlanticusvar. atlanticus, C. atlanticusvar. neapolitanusand C. atlanticusvar. skeleton. The new taxon belongs to the subgenus Chaetoceros, which is characterized by numerous granular chloroplasts in the setae as well as the body of the cell and a rimoportula on each valve, and to the section Atlantica, which is characterized by a long tubular external rimoportula at the centre of each valve, terminal and intercalary setae differing in length and direction, and wide hexagonal apertures. Chaetoceros atlanticusvar. koreanusshares its general morphological characteristics with three closely related species, but has rows of small poroids forming a grid pattern, a tetragonal cross-section, and perpendicular costae on the setae. These features are not seen in the three most closely related taxa, but are sometimes found in other species included in the subgenus Chaetoceros. We therefore designate C. atlanticusvar. koreanusas a new variety of C. atlanticus.