To evaluate the associations between Diagnostic and Statistical Manual of Mental Disorders, 5th Edition (DSM-5) Alternative Model of Personality Disorder traits and domains and categorically diagnosed narcissistic personality disorder (NPD) and borderline personality disorder (BPD), respectively, 238 inpatient and outpatient participants who were consecutively admitted to the Clinical Psychology and Psychotherapy Unit of San Raffaele Hospital inMilan, Italy, were administered the Personality Inventory for DSM-5 (PID-5) and the Structured Clinical Interview for DSM-IV Axis II Personality Disorders (SCID-II). Based on SCID-II, the participants were assigned to the following groups: a) NPD (n = 49), b) BPD (n = 32), c) any other PD (n = 91), and d) no PD (n = 63). Emotional lability, separation insecurity, depressivity, impulsivity, risk taking, and hostility were significantly associated with BPD diagnosis. Attention seeking significantly discriminated participants who received an SCID-II categorical NPD diagnosis. Separation insecurity, impulsivity, distractibility, and perceptual dysregulation were the DSM-5 traits that significantly discriminated BPD participants. Domain-level analyses confirmed and extended trait-level findings.