One of the most potent protective factors against psychiatric symptoms after military trauma is perceived social support. Although group psychotherapy has been linked with increasing social support, no research has evaluated which therapeutic mechanisms are associated with this increase beyond symptom reduction. We investigated which interpersonal therapeutic factors were related to changes in social support, beyond posttraumatic stress disorder (PTSD) symptom reduction. Participants were 117 veterans in a multimodal outpatient group psychotherapy treatment designed to reduce PTSD symptoms and interpersonal difficulties. Generally, therapeutic factors were related to improvements in social support from baseline to posttreatment beyond the effects of PTSD symptom reduction. Specifically, social learning was associated with changes in appraisal support, secure emotional expression was associated with changes in tangible support, and neither was associated with changes in belonging support. Depending on the goals of treatment, understanding these variations are important so clinicians and researchers can appropriately design and target their interventions to facilitate desired changes.