ABSTRACT Background: Dissociation is commonly reported by individuals who meet criteria for posttraumatic stress disorder (PTSD) and complex posttraumatic stress disorder (CPTSD). However, the association between the age of trauma exposure, dissociation, and CPTSD is not well understood. This study aimed to test whether dissociation mediated the relationship between the developmental stage of trauma exposure and CPTSD symptoms. Method: A nationally representative sample from Ireland (N = 1020) completed self-report measures on trauma exposure in childhood, adolescence, adulthood, current trauma symptomatology, and dissociation symptoms. A mediation analysis was conducted. Results: Childhood, adolescent, and adulthood trauma exposure were all related to dissociation and ICD-11 CPTSD symptom clusters. Dissociation mediated the effect of developmental stage of trauma exposure on PTSD and disturbances in self-organization(DSO). The direct and indirect effect models provided the best fit of the data. Childhood trauma exposure was the only developmental stage that was directly associated with both PTSD and DSO symptoms in our sample. Conclusion: Dissociation mediates the relationship between reported trauma exposure and the presence of ICD-11 CPTSD symptom clusters, and this relationship appears at its strongest when trauma occurs in childhood. CPTSD interventions should also promote dissociation management to aid recovery from this debilitating condition.