Five species of the genus Pseudodiaptomus, P. inopinus, P. poplesia, P. marinus, P. nihonkaiensis, and P. sp. occur in Korea nearshore waters. Although the four species except for P. sp. have been classified into Lobus and Ramosus groups, two for each group, based on morphological characters, this classification had yet to be confirmed by molecular characters. Here, we determined molecular characters and phylogenetic relationships of the five species in order to evaluate the morphology-based groupings and the species identifications. For this, a 625-bp DNA region of the mitochondrial gene cytochrome oxidase subunit I (mtCOI) was sequenced and compared among the species. Intraspecific variation of the sequences is less than 0.6%, while interspecific variation ranges from 17.6–26.7%, indicating every species, including P. sp., is a genetically distinct, valid species. Phylogenetic trees of the mtCOI DNA reveal that the Lobus-group species including P. inopinus and P. poplesia form a well-supported clade and that P. sp. belongs to this group. On the other hand, the Ramosus group clade consisting of P. marinus and P. nihonkaiensis is not well supported by bootstrap analyses, suggesting that further evaluation of the validity of this group assignment is needed.