Veterans with PTSD often have substantial interpersonal difficulties and low levels of social support, which puts them at increased risk of mortality, but few treatments address global social impairment for veterans with PTSD. This study is a pilot randomized trial of Acceptance and Commitment Therapy to Improve Social Support for Veterans with PTSD (ACT-SS), a psychotherapy that targets social avoidance and eroded social relationships, compared to Person-Centered Therapy (PCT), a non-directive psychotherapy. Participants were randomized to twelve sessions of either ACT-SS (n = 21) or PCT (n = 19). The results showed that veterans with PTSD had high ratings of satisfaction for both treatments. Contrary to the PCT group, participants in the ACT-SS group showed a significant improvement in the quality of social relationships, engagement in social and leisure activities, and PTSD symptoms from the baseline assessment to the end of treatment and a three-month follow-up. Veterans in the ACT-SS group, but not the PCT group, also showed significant improvements in mindfulness and valued living and a reduction in experiential avoidance from baseline to the end of treatment, with sustained improvements in valued living at the three-month follow-up. Overall, the present study demonstrated the feasibility, acceptability, and positive preliminary outcomes of ACT-SS for veterans with PTSD.