Complex posttraumatic stress disorder (CPTSD) was added to the diagnostic nomenclature in the 11th revision of the International Classification of Diseases (ICD-11). Although considerable evidence exists supporting the construct validity of CPTSD, the distinguishability of CPTSD symptoms from those of borderline personality disorder (BPD) has been questioned. The present study examined the discriminant validity of CPTSD and BPD symptoms among a trauma-exposed population sample from the United Kingdom (N = 546). Participants completed self-report measures of CPTSD and BPD symptoms, and their latent structure was assessed using exploratory structural equation modeling (ESEM). A three-factor model with latent variables reflecting PTSD, disturbances in self-organization (DSO), and BPD symptoms provided the best fit of the data, χ2(399, N = 546) = 1,650, p < .001; CFI = .944; TLI = .930; RMSEA = .077, 90% CI [.073, .081]. We identified multiple symptoms distinctive to individual constructs (e.g., disturbed relationships and suicidality) as well as symptoms shared across the constructs (e.g., affective dysregulation). The PTSD, β = .24; DSO, β = .23; and BPD, β = .27, latent variables were positively and significantly associated with childhood interpersonal trauma. The current findings support the discriminant validity of CPTSD and BPD symptoms and highlight various phenomenological signatures of each construct as well as demonstrate how these constructs share important similarities in symptom composition and exogenous correlates.