The coronavirus disease 2019 (COVID-19) pandemic caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) presents a global health emergency that is in urgent need of intervention1-3. The entry of SARS-CoV-2 into its target cells depends on binding between the receptor-binding domain (RBD) of the viral spike protein and its cellular receptor, angiotensin-converting enzyme 2 (ACE2)2,4-6. Here we report the isolation and characterization of 206 RBD-specific monoclonal antibodies derived from single B cells from 8 individuals infected with SARS-CoV-2. We identified antibodies that potently neutralize SARS-CoV-2; this activity correlates with competition with ACE2 for binding to RBD. Unexpectedly, the anti-SARS-CoV-2 antibodies and the infected plasma did not cross-react with the RBDs of SARS-CoV or Middle East respiratory syndrome-related coronavirus (MERS-CoV), although there was substantial plasma cross-reactivity to their trimeric spike proteins. Analysis of the crystal structure of RBD-bound antibody revealed that steric hindrance inhibits viral engagement with ACE2, thereby blocking viral entry. These findings suggest that anti-RBD antibodies are largely viral-species-specific inhibitors. The antibodies identified here may be candidates for development of clinical interventions against SARS-CoV-2.