This study identifies a human monoclonal antibody, S309, that potently neutralizes both SARS-CoV and SARS-CoV-2. S309 engages an epitope containing a conserved glycan within the *Sarbecovirus* subgenus, without competing with receptor binding. The antibody's mechanism of action involves recruiting effector functions, such as antibody-dependent cell cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). S309 also enhances neutralization when combined with other antibodies, suggesting potential cocktails for prophylaxis or treatment of severe disease. The findings highlight the potential of S309 and antibody cocktails for controlling the SARS-CoV-2 pandemic.This study identifies a human monoclonal antibody, S309, that potently neutralizes both SARS-CoV and SARS-CoV-2. S309 engages an epitope containing a conserved glycan within the *Sarbecovirus* subgenus, without competing with receptor binding. The antibody's mechanism of action involves recruiting effector functions, such as antibody-dependent cell cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). S309 also enhances neutralization when combined with other antibodies, suggesting potential cocktails for prophylaxis or treatment of severe disease. The findings highlight the potential of S309 and antibody cocktails for controlling the SARS-CoV-2 pandemic.