Objective: Tonic immobility (TI) and peritraumatic dissociation (PD) are common peritraumatic reactions associated with psychopathology following trauma. The present study aimed to test whether TI and PD mediated the relationship between perceived threat during an episode of rocket shelling and subsequent posttraumatic stress symptoms. Methods: In a prospective study among 226 Israeli civilians, data were collected both during rocket shelling, between May 14, 2021, until ceasefire on May 21, 2021 (T1) and 1 to 2 months after ceasefire (T2). Measures included the Tonic Immobility Scale, Peritraumatic Dissociative Experiences Questionnaire, and PTSD Checklist for DSM-5. Four mediation models were applied for each posttraumatic stress symptom cluster. Results: Findings showed that a substantial proportion of participants had developed posttraumatic stress disorder (PTSD) symptoms at the time of follow-up (18.8%). Both TI and PD fully mediated the relationship between perceived threat and symptoms of intrusion, avoidance, and negative alterations in mood and cognition, but only PD mediated the relationship with alterations in arousal and reactivity. Conclusions: The present findings suggest that TI and PD may serve as mechanisms underlying the link between individuals’ appraisals of threat during the peritraumatic phase and subsequent PTSD symptomatology. Future research should seek to replicate the present findings before any conclusions can be drawn. In particular, the association between PD and arousal and reactivity symptoms should be further explored, given that it might be multifaceted in nature.