Gönen Stream, which is about 134 km in length, arises from the Ida Mountains near Balıkesir and flows out near Misakça to the Sea of Marmara. In order to determine the freshwater amphipod species of Gönen Stream, 8 seasonal samplings were conducted between June 2010 and June 2012. Samplings were made at 16 stations located between the upper and lower parts of the stream. As a result of the study, 3 amphipod species were found, one of which is new to science: Gammarus gonensis sp. Nov. The newly described species belongs to the Gammarus pulex-group. Gammarus gonensis resembles G. Uludagi G. Karaman, 1975 by having long curled setae on the second gnathopods of the males. It differs from G. Uludagi by having many long setae along the anterior margins of pereopods 5 to 7 in both sexes, and by having less setose peduncular segments of antenna 2. Some other characteristic features of the newly described species are also discussed, and detailed drawings of the extremities of holotype and allotype specimens are presented.