The Yale-Brown Obsessive-Compulsive Scale Modified for Body Dysmorphic Disorder for Adolescents (BDD-YBOCS-A) is a clinician-rated measure of BDD symptom severity in youth. Despite widespread use in both research and clinical practice, its psychometric properties have not been formally evaluated. The current study examined the factor structure, reliability, validity, and sensitivity to change of the BDD-YBOCS-A in 251 youths with BDD attending two specialist clinics. A principal component analysis identified two factors, explaining 56% of the variance. The scale showed good internal consistency (Cronbach’s alpha = 0.87) and adequate convergent and divergent validity. In a subgroup of participants receiving BDD treatment (n = 175), BDD-YBOCS-A scores significantly decreased over time, demonstrating sensitivity to change. BDD-YBOCS-A change scores over treatment were highly correlated with severity changes measured by the Clinical Global Impression – Severity scale (r = .84). The study provides empirical support for the use of the BDD-YBOCS-A in children and adolescents with BDD.