Difficulties with emotion regulation and behavioral instability, including impulsive aggression, are seen as core dimensions underlying borderline personality disorder (BPD). Although both BPD and antisocial personality disorder (ASPD) are associated with impulsivity and aggressive behavior, difficulties regulating emotions may be associated uniquely with BPD and may explain distinctive associations between BPD and aggression. This study was designed to examine the unique prospective associations between BPD symptoms at baseline, difficulties with emotion regulation and trait impulsivity, and psychological and physical aggression (both perpetration and victimization) over the course of a year after controlling for ASPD symptoms in a mixed clinical and community sample of adults (N = 150). Results of a multivariate path analysis demonstrated that associations between BPD symptoms at baseline and later psychological and physical aggression were fully mediated by difficulties with emotion regulation. Although BPD symptoms also predicted trait impulsivity, impulsivity did not predict aggression after controlling for emotion dysregulation. ASPD symptoms were directly associated with physical assault perpetration and victimization but were not associated with emotion dysregulation, impulsivity, or psychological aggression. These findings suggest that although both BPD and ASPD are associated with aggressive behaviors, associations between BPD symptoms and aggression are mediated uniquely by difficulties regulating emotions.