Borderline personality disorder (BPD) has a core embodied in affective and behavioral dysregulations, impulsivity, and relational disturbance. Clinical presentation might be heterogeneous due to a combination of different symptoms listed in the DSM-5. Clinical diagnosis and assessment of the severity of manifestations might be improved through the administration of structured interviews such as the Borderline Personality Disorder Severity Index, 4th edition (BPDSI-IV). The psychometric properties of the Italian version of the BPDSI-IV were examined for the first time in 248 patients affected by BPD and 113 patients affected by bipolar disorder, proving to be a valid and accurate instrument with good internal consistency and high accuracy. The Italian version also demonstrates significant validity in the discrimination between these clinical groups (p < 5001).