Background: The International Trauma Interview (ITI) is a structured clinician-administered measure developed to assess posttraumatic stress disorder (PTSD) and complex PTSD (CPTSD) as defined in the 11th version of the International Classification of Diseases (ICD- 11). This study aimed to investigate a psychometric evaluation of the ITI and to finalise the English language version. Method: The latent structure, internal consistency, interrater agreement, and convergent and discriminant validity were evaluated with data from a convenience sample, drawn from an existing research cohort, of 131 trauma exposed participants from the United Kingdom reporting past diagnosis for PTSD or who had screened positively for traumatic stress symptoms. A range of self-report measures evaluating depression, panic, insomnia, dissociation, emotion dysregulation, negative cognitions about self, interpersonal functioning and general wellbeing were completed. Results: Confirmatory factor analysis supported an adjusted second-order two-factor model of PTSD and disturbances in self-organisation (DSO) symptoms, allowing affect dysregulation to also load onto the PTSD factor, over alternative models. The ITI scores showed acceptable internal consistency, and interrater reliability was strong. Findings for convergent and discriminant validity were mostly as predicted for PTSD and DSO domains. Correlations with the ITQ were good but coefficients for the level of agreement of PTSD diagnosis and CPTSD diagnosis between the ITI and the ITQ were weaker, and item level agreement was variable. Conclusion: Results provide support for the reliability and validity of the ITI as a measure of ICD-11 PTSD and CPTSD. Final revisions of the ITI are described.