The study aimed to engineer a viral antigen for respiratory syncytial virus (RSV) that provides greater protection than currently available vaccines. The focus was on antigenic site Θ, a metastable site specific to the prefusion state of the RSV fusion (F) glycoprotein, which is targeted by potent RSV-neutralizing antibodies. Structure-based design yielded stabilized versions of RSV F that maintained antigenic site Θ when exposed to extreme pH, osmolality, and temperature. Six RSV F-crystal structures provided atomic-level insights into how introduced cysteine residues and filled hydrophobic cavities improved stability. Immunization with site Θ-stabilized variants of RSV F in mice and macaques elicited levels of RSV-specific neutralizing activity many times the protective threshold. The study highlights the potential of structure-based design to enhance vaccine efficacy and the importance of selecting appropriate target sites for eliciting protective immune responses.The study aimed to engineer a viral antigen for respiratory syncytial virus (RSV) that provides greater protection than currently available vaccines. The focus was on antigenic site Θ, a metastable site specific to the prefusion state of the RSV fusion (F) glycoprotein, which is targeted by potent RSV-neutralizing antibodies. Structure-based design yielded stabilized versions of RSV F that maintained antigenic site Θ when exposed to extreme pH, osmolality, and temperature. Six RSV F-crystal structures provided atomic-level insights into how introduced cysteine residues and filled hydrophobic cavities improved stability. Immunization with site Θ-stabilized variants of RSV F in mice and macaques elicited levels of RSV-specific neutralizing activity many times the protective threshold. The study highlights the potential of structure-based design to enhance vaccine efficacy and the importance of selecting appropriate target sites for eliciting protective immune responses.