Introduction: Social support is known to buffer posttraumatic stress disorder (PTSD) symptoms, but the stress-buffering properties of striving to help and support others (compassionate goals) have received less attention. Recent research suggests that compassionate goals shape social support processes and dampen stress responses following social threat, but their relevance to trauma have not been examined. The present study tested whether, in the aftermath of a university mass shooting, compassionate goals concurrently and prospectively predicted lower PTSD symptoms indirectly via higher social support, and whether this mediation would be stronger with higher proximity to the trauma. Method: Participants (N = 369) completed measures four months post-shooting (Time 1), and a subset (n = 85) repeated outcome variables at eight months (Time 2). Results: As hypothesized, there was a significant moderated mediation, with trauma proximity moderating the indirect effect of compassionate goals on PTSD symptoms at Time 1 (B = −0.88, SE = 0.28, 95% CI [−1.47, −0.37]) and prospectively at Time 2 (B = −1.12, SE = 0.77, 95% CI [−3.16, −0.03]), although not when controlling for Time 1 symptoms. The indirect effects were greatest for those with higher trauma proximity, suggesting particular relevance for individuals most at risk for trauma-related difficulties. Discussion: These findings suggest the need for further research into compassionate goals and strategies for fostering them, particularly in the context of social or community traumas.