Current COVID-19 vaccines and many clinical diagnostics are based on the structure and function of the SARS-CoV-2 spike ectodomain. Using hydrogen-deuterium exchange monitored by mass spectrometry, we have uncovered that, in addition to the prefusion structure determined by cryo-electron microscopy, this protein adopts an alternative conformation that interconverts slowly with the canonical prefusion structure. This new conformation-an open trimer-contains easily accessible receptor-binding domains. It exposes the conserved trimer interface buried in the prefusion conformation, thus exposing potential epitopes for pan-coronavirus antibody and ligand recognition. The population of this state and kinetics of interconversion are modulated by temperature, receptor binding, antibody binding, and sequence variants observed in the natural population. Knowledge of the structure and populations of this conformation will help improve existing diagnostics, therapeutics, and vaccines.