Despite growing research on the impact of borderline personality pathology (BPP) on treatment outcomes for emotional disorders among adults, no studies have examined this question in adolescents. Moreover, no studies have examined mediators of the relation between BPP and changes in mood and anxiety symptoms during treatment. This study examined the impact of BPP on treatment outcomes in 141 adolescents in psychiatric residential treatment, as well as the mediating role of change in emotion regulation (ie, adaptive responses to emotions) in the relation between baseline BPP and improvements in psychiatric symptoms during treatment. Participants completed questionnaires assessing emotion regulation and depression, anxiety, and posttraumatic stress disorder symptoms at baseline and posttreatment. Although BPP was not directly associated with the magnitude of change in psychiatric symptoms from baseline to posttreatment, it was positively associated with greater improvement in emotion regulation during treatment. Furthermore, results revealed significant indirect effects of BPP on improvements in all psychiatric symptoms through improvement in emotion regulation.