α₄β₇ integrin–expressing CD4⁺ T cells preferentially traffic to gut-associated lymphoid tissue (GALT) and have a key role in HIV and simian immunodeficiency virus (SIV) pathogenesis. We show here that the administration of an anti-α₄β₇ monoclonal antibody just prior to and during acute infection protects rhesus macaques from transmission following repeated low-dose intravaginal challenges with SIV_mac251. In treated animals that became infected, the GALT was significantly protected from infection and CD4⁺ T cell numbers were maintained in both the blood and the GALT. Thus, targeting α₄β₇ reduces mucosal transmission of SIV in macaques.