Objective The 11th version of the International Classification of Diseases (ICD‐11) has proposed two related trauma diagnoses: Post‐traumatic stress disorder (PTSD) and Complex PTSD (CPTSD). Using a newly developed, disorder‐specific measure of PTSD and CPTSD called the International Trauma Questionnaire (ITQ) the current study will (i) assess the factorial validity of ICD‐11 PTSD and CPTSD; (ii) provide the first test of the discriminant validity of these constructs; and (iii) provide the first comparison of ICD‐11, and Diagnostic and Statistical Manual, Fifth Edition (DSM‐5), PTSD diagnostic rates using disorder‐specific measures. Method ICD‐11 and DSM‐5 PTSD‐specific measures were completed by a British clinical sample of trauma‐exposed patients (N = 171). The structure and validity of ICD‐11 PTSD and CPTSD were assessed by means of factor analysis and assessing relationships with criterion variables. Results Diagnostic rates under ICD‐11 were significantly lower than those under DSM‐5. A two‐factor second‐order model reflecting the distinction between PTSD and CPTSD best represented the data from the ITQ; and the PTSD and CPTSD factors differentially predicted multiple psychological variables. Conclusion The factorial and discriminant validity of ICD‐11 PTSD and CPTSD was supported, and ICD‐11 produces fewer diagnostic cases than DSM‐5.