This study evaluated whether tonic immobility mediates the relations between perceived inescapability, peritraumatic fear, and posttraumatic stress disorder (PTSD) symptom severity among sexual assault survivors. Female undergraduates (N = 176) completed questionnaires assessing assault history, perceived inescapability, peritraumatic fear, tonic immobility, and PTSD symptoms. Results indicated that tonic immobility fully mediated relations between perceived inescapability and overall PTSD symptom severity, as well as reexperiencing and avoidance/numbing symptom clusters. Tonic immobility also fully mediated the relation between fear and reexperiencing symptoms, and partially mediated relations between fear and overall PTSD symptom severity, and avoidance/numbing symptoms. Results suggest that tonic immobility could be one path through which trauma survivors develop PTSD symptoms. Further study of tonic immobility may inform our ability to treat trauma victims.