A new species of freshwater amphipod, Gammarus obruki sp. nov., collected from Inderesi Cave, Bartin Province, Turkey, is described and illustrated. The new species belongs to the Gammarus pulex group. The most discriminant characters of this species are the presence of prolonged extremities, a very long antenna 1 (up to 52 segmented flagellum), densely setose fifth peduncle and flagellar segments of antenna 2, and a fourth peduncle segment that has no long setae. A detailed morphological description and illustrations of the new species are provided and differences from related species are discussed.