Introduction: Family support and parental invalidation of their child’s emotions are important in the treatment of adolescents with nonsuicidal self-injury (NSSI). Aims: The aims of this study were to investigate the impact of a therapist-supported parental course included in the internet-delivered emotion regulation individual therapy for adolescents (IERITA) with NSSI. Methods: The current study was a 3-site randomized controlled trial comparing the parental course of IERITA with treatment as usual including caregivers to adolescents (13-17 years of age) fulfilling the tentative diagnostic criteria for NSSI disorder and presenting with recent NSSI. IERITA includes 6 modules for the parents and 11 modules for the adolescent participant delivered over the course of 12 weeks. Parents’ perceived ability to cope with and respond effectively to their children’s negative emotions was assessed with the adolescent version of the Coping with Children’s Negative Emotions Scale at post-treatment and at 3-month post-treatment. Data was analyzed using linear mixed-effects models and effect sizes were calculated using Cohen’s d. Results: In total, 166 families (treatment-responsible caregiver: mean [SD] age, 46.3 [5.1] years; 87% female) were randomized to IERITA (n=84) or treatment as usual (n=82). Parents completed a mean (SD) of 5.5 modules out of 6. IERITA was statistically significantly more efficacious than TAU in improving parental distress (d=0.33, 95% CI: 0.05-0.61) and punitive (d=0.18, 0.03-0.33) and minimizing (d=0.69, 0.39-0.99) responses to adolescents’ expressions of negative emotions. Conclusion: The IERITA parental course is a brief, scalable, and transportable intervention that improved parental ability to respond effectively to their children's emotions. The author will present the results and discuss the role of changes in parental reactions and behaviors in treatment outcomes among adolescents with NSSI and other maladaptive behaviors.