Abstract: Objective: Posttraumatic stress disorder (PTSD) and more complex posttraumatic symptomatology (i.e., dissociative PTSD [D-PTSD] and complex PTSD [CPTSD]) are differently described in the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5) and the International Classification of Diseases (11th ed.; ICD-11). Although the choice of system may affect diagnostic prevalence rates and treatment outcome, less is known about the more complex symptoms and their associated risk factors. Method: To investigate both D-PSTD and CPTSD in Northern Irish military veterans (n = 436) using latent class analysis and associated risk factors to gain a deeper understanding of the potential implications of applying one diagnostic system instead of the other. Results: The latent class analyses revealed a DSM-5 four-class solution and an ICD-11 five-class solution with both a highly symptomatic D-PTSD class (27.52%) and CPTSD class (27.9%) identified. Similar associations with risk factors were found across the diagnostic systems (e.g., medium to strong effect sizes for prior traumatic exposure, depression, anxiety, dissociation, and alcohol use). Conclusions: Both D-PTSD and CPTSD appear to be highly prevalent among Northern Irish veterans, and interestingly, similar effect sizes were found for the investigated risk factors for highly symptomatic groups across diagnostic systems. Research is needed to determine the generalizability of the results.