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.