Borderline Personality Disorder (BPD) is associated with emotion dysregulation. The emotion regulation strategies people adopt may depend on one’s belief about emotion as either fixed or changeable (termed “implicit theory”). We test this experimentally by modifying beliefs about emotions using virtual reality (VR). A sample of 29 adolescents (ages 14–17 years) were recruited from two adolescent inpatient units for an uncontrolled pilot trial of a new brief intervention. We measured BPD traits, beliefs about emotion, treatment preference, cognitive reappraisal and rumination, before the VR game and 14–31 days later. Adolescents with higher levels of BPD traits endorsed fixed beliefs of emotion and reported higher levels of rumination and lower levels of cognitive reappraisal. After a one-time message delivered via VR, adolescents evidenced an increase in belief that their emotions were changeable. These findings suggest beliefs about emotions may have an important role in interventions for adolescents with BPD traits.