Respiratory syncytial virus (RSV) is the leading cause of hospitalisation for children under 5 years of age. We sought to engineer a viral antigen that provides greater protection than currently available vaccines and focused on antigenic site φ, a metastable site specific to the prefusion state of the RSV fusion (F) glycoprotein, as this site is targeted by extremely potent RSV-neutralizing antibodies. Structure-based design yielded stabilized versions of RSV F that maintained antigenic site φ when exposed to extremes of pH, osmolality, and temperature. Six RSV F crystal structures provided atomic-level data on how introduced cysteine residues and filled hydrophobic cavities improved stability. Immunization with site φ—stabilized variants of RSV F in mice and macaques elicited levels of RSV-specific neutralizing activity many times the protective threshold.