Background <p>Borderline Personality Disorder (BPD) is a severe psychiatric condition frequently associated with Substance Use Disorder (SUD), with high rates of comorbidity and poor clinical outcomes. Impulsivity and emotional dysregulation are core features shared by both disorders, suggesting common underlying mechanisms. However, less is known about the role of additional factors—such as alexithymia, coping strategies, and quality of life in contributing to dual diagnosis and SUD severity in individuals with BPD. This study aims to: (1) examine the clinical and demographic variables in a sample of patients with BPD comorbid with SUD, in comparison with a group of patients with BPD alone, and (2) identify clinical and psychopathological factors related to the presence of SUD and severity of SUD symptoms in the whole sample (BPD with and without SUD).</p> Methods <p>Seventy-five outpatients diagnosed with BPD were recruited and divided into two groups: BPD with comorbid SUD (n = 43) and BPD without SUD (n = 32). Diagnoses were established using SCID-5-CV and SCID-5-PD. Participants were assessed using validated instruments measuring BPD severity (BPDSI-IV), impulsivity (BIS-11), alexithymia (TAS-20), coping strategies (CISS), addiction vulnerability and severity (ABQ), quality of life (SAT-P), and functioning (SOFAS). Group differences were analyzed using chi-square and ANCOVA. Hierarchical logistic and linear regressions were conducted to identify factors associated with the SUD presence and severity.</p> Results <p>Patients with BPD and SUD were more frequently male and showed earlier BPD onset, greater symptom severity, higher impulsivity, higher vulnerability to addiction, greater alexithymia, and more use of task-oriented coping strategies. Logistic regression showed that impulsivity and vulnerability to SUD were significantly associated with comorbid SUD, while alexithymia reduced the effect of gender. Motor impulsiveness independently contributed to the presence of dual diagnosis. Linear regression analyses indicated that alexithymia and impulsivity were variables most strictly associated with SUD severity, jointly explaining approximately 60% of the variance. Specifically, difficulty describing feelings and externally oriented thinking contributed to a greater SUD severity.</p> Conclusions <p>Impulsivity and alexithymia appear to be key transdiagnostic factors underlying both the presence and severity of SUD in individuals with BPD. These findings highlight the importance of targeting deficits in emotional processing and impulse control in clinical interventions to improve outcomes in this clinical population.</p>