The morphology of 26 species of the genus Tripos was studied among a total of 51 species of this genus, which were taken using $20{mu}m$ net samples from June 2006 to January 2011 at 14 stations around Jeju Island. Of these, 23 species were first described as new records in Korean waters and 3 species were redescribed. Short descriptions and synonyms are given for each species in the present study. The dinoflagellates of family Ceratiaceae include marine and freshwater species, belonged to originally the genus Ceratium. Recently, this genus were divided into a new combination with the genus Tripos, which contains the marine species, and the original genus Ceratium, including the freshwater species. We used the Tripos genus for the marine species in this study as a valid name.