Objective: The prevalence, construct validity, risk factors and psychopathological correlates associated with ICD-11 posttraumatic stress disorder (PTSD) and complex PTSD (CPTSD) as measured by the International Trauma Questionnaire for Children and Adolescents (ITQ-CA) were assessed in a sample of young people from Northern Ireland. Method: Participants were trauma-exposed 11-19-year-olds (N = 507) who participated in the Northern Ireland Youth Wellbeing Prevalence Survey (YWS-NI, 2020). Factor mixture modelling (FMM) was used to test the latent structure of the ITQ-CA. Risk-factors and psychopathological correlates associated with latent class membership, and ICD-11diagnostic status, were also investigated. Results: More participants met the ITQ-CA criteria for CPTSD (3.4%, n = 44) than PTSD (1.5%, n = 19). A second-order FMM comprising a 'partial-PTSD class', a 'CPTSD class', a 'DSO class' and a 'low symptom endorsement class' was the best-fitting model. Younger age and cumulative trauma were risk factors for all trauma classes. Female gender and two or more violent traumas were significant predictors of the 'PTSD' and 'CPTSD' classes, while single sexual trauma was a significant predictor of the 'DSO' and 'CPTSD' classes. Two or more sexual traumas was a unique predictor of 'CPTSD class', while two or more vicarious traumas was a unique predictor of 'DSO class'. The 'CPTSD' class displayed the most notable comorbidity. Conclusions: Findings indicate that CPTSD may be more prevalent than PTSD in children and young people. Support for the ICD-11 conceptualisation of CPTSD as representing a unique diagnostic construct was supported using FMM, with findings indicating trauma symptom class-specific risk profiles.