Fermi LAT Collaboration, M Ackermann, M Ajello, J Ballet, G Barbiellini, D Bastieri, A Belfiore, R Bellazzini, B Berenji, R D Blandford, E D Bloom, E Bonamente, A W Borgland, J Bregeon, M Brigida, P Bruel, R Buehler, S Buson, G A Caliandro, R A Cameron, P A Caraveo, E Cavazzuti, C Cecchi, Ö Çelik, E Charles, S Chaty, A Chekhtman, C C Cheung, J Chiang, S Ciprini, R Claus, J Cohen-Tanugi, S Corbel, R H D Corbet, S Cutini, A de Luca, P R den Hartog, F de Palma, C D Dermer, S W Digel, E do Couto e Silva, D Donato, P S Drell, A Drlica-Wagner, R Dubois, G Dubus, C Favuzzi, S J Fegan, E C Ferrara, W B Focke, P Fortin, Y Fukazawa, S Funk, P Fusco, F Gargano, D Gasparrini, N Gehrels, S Germani, N Giglietto, F Giordano, M Giroletti, T Glanzman, G Godfrey, I A Grenier, J E Grove, S Guiriec, D Hadasch, Y Hanabata, A K Harding, M Hayashida, E Hays, A B Hill, R E Hughes, G Jóhannesson, A S Johnson, T J Johnson, T Kamae, H Katagiri, J Kataoka, M Kerr, J Knödlseder, M Kuss, J Lande, F Longo, F Loparco, M N Lovellette, P Lubrano, M N Mazziotta, J E McEnery, P F Michelson, W Mitthumsiri, T Mizuno, C Monte, M E Monzani, A Morselli, I V Moskalenko, S Murgia, T Nakamori, M Naumann-Godo, J P Norris, E Nuss, M Ohno, T Ohsugi, A Okumura, N Omodei, E Orlando, M Ozaki, D Paneque, D Parent, M Pesce-Rollins, M Pierbattista, F Piron, G Pivato, T A Porter, S Rainò, R Rando, M Razzano, A Reimer, O Reimer, S Ritz, R W Romani, M Roth, P M Saz Parkinson, C Sgrò, E J Siskind, G Spandre, P Spinelli, D J Suson, H Takahashi, T Tanaka, J G Thayer, J B Thayer, D J Thompson, L Tibaldo, M Tinivella, D F Torres, G Tosti, E Troja, Y Uchiyama, T L Usher, J Vandenbroucke, G Vianello, V Vitale, A P Waite, B L Winer, K S Wood, M Wood, Z Yang, S Zimmer, M J Coe, F Di Mille, P G Edwards, M D Filipović, J L Payne, J Stevens, M A P Torres

Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.