Posttraumatic stress disorder (PTSD) is one of the most common psychiatric disorders among veterans returning from Iraq and Afghanistan. Little research has examined variables that may mediate the relation between PTSD and aspects of social functioning, such as relationship satisfaction and family functioning. In this cross-sectional study, a total of 164 veterans who were seeking VA primary care or mental health care within one year after returning from Iraq and/or Afghanistan were screened for PTSD and completed a series of questionnaires that assessed social functioning, coping, and life satisfaction. Results showed that the 86 (52%) veterans who screened positive for PTSD reported greater difficulties in their relationships with romantic partners, less cohesion in their families, less social support, poorer social functioning, and lower life satisfaction compared to other treatment-seeking veterans. Less social support from the community, excessive worry, decreased acceptance of change, and lower availability of secure relationships mediated the association between PTSD and poor social functioning. The relation between PTSD and lower partner satisfaction was mediated by greater cognitive social avoidance and lower availability of secure relationships. These results suggest that psychotherapeutic interventions that address these mediating variables may help improve social functioning in treatment-seeking veterans with PTSD.