The American Psychiatric Association and the World Health Organization provide distinct trauma-based diagnoses in the fifth edition of the Diagnostic and Statistical Manual (DSM-5), and the forthcoming 11th version of the International Classification of Diseases (ICD-11), respectively. The DSM-5 conceptualizes posttraumatic stress disorder (PTSD) as a single, broad diagnosis, whereas the ICD-11 proposes two "sibling" disorders: PTSD and complex PTSD (CPTSD). The objectives of the current study were to: (a) compare prevalence rates of PTSD/CPTSD based on each diagnostic system; (b) identify clinical and behavioral variables that distinguish ICD-11 CPTSD and PTSD diagnoses; and (c) examine the diagnostic associations for ICD-11 CPTSD and DSM-5 PTSD. Participants in a predominately female clinical sample (N = 106) completed self-report scales to measure ICD-11 PTSD and CPTSD, DSM-5 PTSD, and depression, anxiety, borderline personality disorder, dissociation, destructive behaviors, and suicidal ideation and self-harm. Significantly more people were diagnosed with PTSD according to the DSM-5 criteria (90.4%) compared to those diagnosed with PTSD and CPTSD according to the ICD-11 guidelines (79.8%). An ICD-11 CPTSD diagnosis was distinguished from an ICD-11 PTSD diagnosis by higher levels of dissociation (d = 1.01), depression (d = 0.63), and borderline personality disorder (d = 0.55). Diagnostic associations with depression, anxiety, and suicidal ideation and self-harm were higher for ICD-11 CPTSD compared to DSM-5 PTSD (by 10.7%, 4.0%, and 7.0%, respectively). These results have implications for differential diagnosis and for the development of targeted treatments for CPTSD.