A new species in the genus Acartia , Acartia nadiensis sp. nov., is described from Fijian coastal waters. This species belongs to the subgenus Odontacartia based on the following morphological features: presence of a rostral filaments, a pointed process on the last prosomite, a serrated terminal spine on female P5, and the absence of a protrusion on the basis of the male right P5. This new species can be differentiated from its congeners by the combination of the absence of a spine on the first segment of the antennules, the short outer seta of female P5, and a medial spine on the exp-2 of the left male P5. Phylogenetic analyses using mitochondrial COI partial sequences show that the new species is distinct from its congeners.