How to best understand theoretically the nature of the relationship between co-occurring PTSD and MDD (PTSD+MDD) is unclear. In a sample of 173 individuals with chronic PTSD, we examined whether the data were more consistent with current co-occurring MDD as a separate construct or as a marker of posttraumatic stress severity, and whether the relationship between PTSD and MDD is a function of shared symptom clusters and affect components. Results showed that the more severe depressive symptoms found in PTSD+MDD as compared to PTSD remained after controlling for PTSD symptom severity. Additionally, depressive symptom severity significantly predicted co-occurring MDD even when controlling for PTSD severity. In comparison to PTSD, PTSD+MDD had elevated dysphoria and re-experiencing - but not avoidance and hyperarousal - PTSD symptom cluster scores, higher levels of negative affect, and lower levels of positive affect. These findings provide support for PTSD and MDD as two distinct constructs with overlapping distress components.