A new species of freshwater Amphipod, Gammarus katagani sp. n., collected from Domanic in Kutahya province, Turkey, is described and illustrated. The new species belongs to the Gammarus balcanicus-group. The presence of an additional row of setae on the last two metasome segments in both sexes is the most diagnostic character for the species. A detailed morphological description and illustrations of the new species are given and differences from related species are discussed.