question: Potently neutralizing SARS-CoV-2 antibodies often target the receptor binding site (RBS) of spike protein but the variability of RBS epitopes hampers broad neutralization of different clades of coronaviruses and emerging drifted viruses. Here, we identified a human RBS antibody that potently neutralizes SARS-CoV and SARS-CoV-2 variants that belong to clade 1 SARS-related coronavirus. X-ray crystallography revealed coordinated recognition by the heavy chain to conserved sites and the light chain to RBS, allowing for the mimicry of ACE2 binding mode. The minimum footprints in the hypervariable region of RBS contributed to the breadth of neutralization, and the activity was further enhanced by IgG3 switching. Eventually, the coordinated binding resulted in broad neutralization of SARS-CoV and emerging SARS-CoV-2 variants of concern. Furthermore, therapeutic treatment in a hamster model provided protection at low dosage. The structural basis for broadly neutralizing activity informs the design of broad spectrum of therapeutics and vaccines.Funding: This work was supported by Japan Agency for Medical Research and Development grant JP19fk0108111 (TH, YT), JP20fk0108298 (TK, TH, KM, YT), JP20am0101093 (KM), JP20ae0101047 (KM), JP20fk0108251 (HS), and JP20am0101124 (YK), by Ministry of Education, Culture, Sports, Science and Technology grant JPMXS0420100119 (KM) and 20H05773 (TH), by The Naito Foundation (TH), and by Joint Usage/Research Center program of Institute for Frontier Life and Medical Sciences, Kyoto University (KM).Conflict of Interest: AS is an employee of Shionogi & Co., Ltd. MO is a CEO, employee, and shareholder of Trans Chromosomics, Inc. These authors acknowledge a potential conflict of interest and attest that the work contained in this report is free of any bias that might be associated with the commercial goals of the company. TO, YA, MO, TH, KM, and YT declare that an intellectual property application has been filed using the data presented in this paper. The other authors declare that they have no competing interests.Ethical Approval: Animal procedures were approved by the Animal Ethics Committee of the National Institute of Infectious Diseases, Japan, and performed in accordance with the guidelines of the Institutional Animal Care and Use Committee. In vitro escape mutation screening experiments for SARSCoV-2 were performed at the Biosafety Level-3 facility of the Research Center for ZoonosisControl, Hokkaido University, and the National Institute of Infectious Diseases following the institutional guidelines.