<br> Gammarus<br> Fabricius, 1775 is the largest genus in the family Gammaridae and most species inhabit freshwater and exhibit high regional endemism. To date, 12 freshwater<br> Gammarus<br> species are known from South Korea. However, considering the complex topography of the Korean Peninsula, the species diversity of freshwater<br> Gammarus<br> may be underestimated.<br> <br> <br> A new species of the genus<br> Gammarus<br> Fabricius, 1775 is described, originating from the Hantan River region, South Korea.<br> Gammarus kyungsooki<br> sp. nov.<br> is morphologically characterised by the absence of calceoli on antenna 2, the presence of 6–7 clusters of long setae on the posterior margin of peduncular article 4 in antenna 2, long straight setae along the posterior margins of merus to carpus in pereopod 3, a 0–2–0 dorsal spine formula on urosomite 1, plumose setae on the outer margin of the outer ramus of uropod 3 and three or more clusters of setae on the outer margin of each telson lobe. Genetic distance analyses, based on mitochondrial cytochrome<br> c<br> oxidase subunit I (COI) and nuclear large-subunit ribosomal RNA (28S) gene sequences, support the distinctiveness of the new species, with interspecific divergences of 18.8%–25.2% (COI) and 1.7%–5.1% (28S).<br>
