Abstract: BACKGROUND: There are two primary competing diagnostic criteria for posttraumatic stress disorder (PTSD); the International Classification of Diseases (ICD) and the Diagnostic and Statistical Manual (DSM). These systems differ in terms of the number and nature of PTSD symptoms, the implied latent structure of the disorder, and associated posttraumatic diagnostic classifications. OBJECTIVE: To investigate the prevalence and concordance of ICD-11th Revision (ICD-11) and DSM-Fifth Edition (DSM-5) PTSD, complex-PTSD (C-PTSD), and dissociative subtype of PTSD (D-PTSD) criteria in a sample of Northern Irish military veterans. METHODOLOGY: Data were collected from a community sample of military veterans living in Northern Ireland (NI). Prevalence rates of ICD-11 PTSD and C-PTSD and DSM-5 PTSD and D-PTSD were estimated using validated self-report measures for each system. Concordance of caseness determined by diagnostic criteria was compared using Cohen's kappa. RESULTS: The DSM-5 algorithm criteria produced significantly higher prevalence estimates of PTSD (39.26%) relative to the ICD-11 algorithm criteria (32.09%). Both C-PTSD and D-PTSD were identified for subgroups within the sample (24.66% and 27.96%, respectively). There was moderate agreement between the diagnostic systems regarding PTSD caseness, and between C-PTSD and D-PTSD caseness. CONCLUSIONS: These findings have implications regarding the measurement of PTSD and associated diagnostic groupings for the wider literature, suggesting that the choice of diagnostic screening framework may influence classification. Both C-PTSD and D-PTSD may be relevant diagnostic considerations for the NI military veteran group.