Arenaviruses exist worldwide and can cause hemorrhagic fever and neurologic disease. A single glycoprotein expressed on the viral surface mediates entry into target cells. This glycoprotein, termed GPC, contains a membrane-associated signal peptide, a receptor-binding subunit termed GP1 and a fusion-mediating subunit termed GP2. Although GPC is a critical target of antibodies and vaccines, the structure of the metastable GP1-GP2 prefusion complex has remained elusive for all arenaviruses. Here we describe the crystal structure of the fully glycosylated prefusion GP1-GP2 complex of the prototypic arenavirus LCMV at 3.5 Å. This structure reveals the conformational changes that the arenavirus glycoprotein must undergo to cause fusion and illustrates the fusion regions and potential oligomeric states.