The validity of ICD-11 Posttraumatic Stress Disorder (PTSD) and Complex PTSD (CPTSD), as measured by the International Trauma Questionnaire (ITQ; Cloitre et al., 2018) has been supported in many factor analytic and mixture modelling studies. There is, however, a paucity of research investigating the latent structure of the ITQ using factor mixture modelling (FMM). FMM was applied to data collected from a nationally representative sample of U.S. adults (N = 1834). FMM results demonstrated strong support for a two-factor second-order model with four qualitatively distinct latent classes: a ‘PTSD class’, a ‘CPTSD class’, a ‘DSO’ (Disturbances in Self-Organisation) class and a ‘low symptoms class’. Sexual abuse increased likelihood of membership to the ‘CPTSD’ (OR = 3.22) and physical abuse decreased likelihood of membership to the ‘PTSD’ (OR=0.51). Trauma exposure in adulthood predicted ‘PTSD’ and ‘CPTSD’ class membership. The ‘CPTSD class’ was characterised by higher levels of psychopathological co-morbidities and poorer psychological wellbeing compared to all other classes. Results provide additional support for the validity of PTSD and CPTSD as measured by the ITQ