Borderline personality disorder (BPD) is a complex psychiatric disorder characterized by emotional instability, impulsivity, and self-destructive behavior. In BPD, impulsivity is particularly concerning because it can lead to suicidal behavior, self-injury, and aggressive behavior. Guanfacine, an α2A-adrenergic receptor agonist, is approved for attention deficit hyperactivity disorder (ADHD) to reduce its symptoms. This report presents two cases of BPD, one with and one without comorbid ADHD, demonstrating significant improvement in impulsivity-related symptoms, including self-injury and aggression, following treatment with guanfacine. Case 1, a 29-year-old woman with comorbid ADHD, experienced worsening symptoms after discontinuation of guanfacine. Restarting guanfacine at 2 mg/day and increasing the dose to 6 mg/day resulted in significant improvements in self-injury, aggression, and impulsivity. Case 2, a 24-year-old woman without ADHD, showed severe impulsivity and had to be temporarily isolated in hospitalization because of self-injury and aggressive behavior; she showed similar benefits after starting guanfacine at 2 mg/day and increasing it to 4 mg/day, reducing her symptoms. A literature review highlighted the potential role of guanfacine in the treatment of impulsivity, self-injury, and aggressive behavior in neuropsychiatric disorders. Guanfacine is thought to improve prefrontal cortex (PFC) dysfunction. Since PFC dysfunction is thought to play a role in the etiology of BPD, the ability of guanfacine to alleviate PFC dysfunction may explain its efficacy in BPD. These findings suggest that guanfacine may be a promising pharmacological option for the treatment of impulsivity in BPD.