The study reports the cryo-EM structure of the prefusion conformation of the 2019-nCoV spike (S) glycoprotein, which is a key target for vaccines, therapeutic antibodies, and diagnostics. The structure reveals that one of the three receptor-binding domains (RBDs) is rotated up in a receptor-accessible conformation. The 2019-nCoV S protein binds to ACE2 with higher affinity than SARS-CoV S, and published SARS-CoV RBD-specific monoclonal antibodies do not bind appreciably to the 2019-nCoV S protein, suggesting limited cross-reactivity. The atomic-resolution structure of 2019-nCoV S will facilitate the rapid development and evaluation of medical countermeasures to address the ongoing public health crisis.The study reports the cryo-EM structure of the prefusion conformation of the 2019-nCoV spike (S) glycoprotein, which is a key target for vaccines, therapeutic antibodies, and diagnostics. The structure reveals that one of the three receptor-binding domains (RBDs) is rotated up in a receptor-accessible conformation. The 2019-nCoV S protein binds to ACE2 with higher affinity than SARS-CoV S, and published SARS-CoV RBD-specific monoclonal antibodies do not bind appreciably to the 2019-nCoV S protein, suggesting limited cross-reactivity. The atomic-resolution structure of 2019-nCoV S will facilitate the rapid development and evaluation of medical countermeasures to address the ongoing public health crisis.