A new species of freshwater Amphipod, Gammarus topkarai sp. nov., collected from Ivriz Creek, Tarsus Province, South Anatolia, is described and illustrated. The new species belongs to Gammarus pulex-group and shows similar characteristic features with Gammarus agrarius G. S. Karaman, 1973. Absence of well developed dorsal elevations on the urosome segments, smaller body length and relatively lower endopod/exopod ratio of third uropod are the most obvious features differ from G. agrarius. A detailed morphological description and illustrations of new species are given and differences from related species are discussed.