Gammarus gageoensis n. sp., a new species of freshwater gammarid, was collected from Gageodo Island, South Korea. This new species is characterized in terms of its morphology by the flagellum of antenna 2 with calceoli, the pereopods 3 and 4 with numerous long curled setae on the posterior margins, the terminal article of the outer ramus in uropod 3, which is longer than the adjacent spines, and the presence of both rami with long simple setae. The morphological characters of the new species that are different to related species are presented in the text. This paper also compares the new species to related species by the partial sequence of the mitochondrial cytochrome c oxidase subunit 1 (CO1) gene. The sequence difference, consisting of a 39-44% difference relative to closely related species, is also large enough to designate this gammarid as a different species from other freshwater gammarid species. This is the first finding of a new species of Korean freshwater gammarid since 1992. Including this new species, a total of 10 species are known to Korea.