Although studies have shown that persons with low socioeconomic position (SEP) are more likely to develop posttraumatic stress disorder (PTSD) than those with high SEP, there has been no systematic attempt to explain this observation. We used data from the Detroit Neighborhood Health Study (DNHS), a longitudinal population-based study of Detroit residents, to explore the relation between SEP and posttraumatic stress (PTS). Our first goal was to assess whether low SEP persons are more likely to develop PTSD than those with high SEP because they are more likely to experience a traumatic event or because they are more vulnerable to psychopathology after the traumatic experience. We then examined additional SEP differences in PTS symptomatology that cannot be evaluated using categorical measures of PTS. Finally, given that social support is one of the strongest predictor of PTS and is thought to buffer against the psychological consequences of stressful events, we investigated the potential modifying role of social support in the relation between SEP and PTS. We found that although low SEP persons are more likely to have experienced assaultive violence than those with high SEP, this did not explain their greater risk of PTSD. Instead, it was the greater burden of concomitant stressful life events among low SEP persons that explained their vulnerability to PTS. An examination of SEP differences in PTS as a continuous variable yielded additional information about PTS symptomatology. We found that differences in PTS score distribution between low and high SEP groups were not only more pronounced in the highest range of scores but also in the middle range of scores, which may be overlooked in categorical conceptualizations of PTS but may also be accompanied by substantial functional impairment. Though not conclusive, there was some evidence that social support reduced socioeconomic vulnerability to PTS. Differences in PTS severity between low and high SEP persons were smaller among those with high social support than among those with low social support. Our findings may inform interventions to reduce socioeconomic disparities in mental health and improve well-being among the most disadvantaged.