Fifty-two species of Ceratium are reported from Vietnamese waters. All species are described and illustrated by light microscopy, and some species were also examined by scanning electron microscopy. The largest diversity of Ceratium was found in South Central Vietnam and its offshore islands, while the lowest diversities were found in North and South east Vietnam, presumably due to low salinity and high turbidity in the delta of the Red River in the north and the outflow of the Mekong River in the south