The aim of this study was to investigate the effectiveness of emotion regulation-based therapy in reducing rumination, cognitive inflexibility, external shame, and negative affect among adolescent girls with symptoms of body dysmorphic disorder (BDD). This quasi-experimental study employed a pretest–posttest control-group design. The study population consisted of adolescent girls aged 16–18 years who exhibited symptoms of BDD in Tabriz during the 2025–2026 academic year. Initially, 300 students from two schools were screened using the Yale-Brown Obsessive-Compulsive Scale for Body Dysmorphic Disorder (Y-BOCS-BDD; Phillips et al., 1997). Students who scored above 20 were identified, and 40 eligible participants who agreed to participate were enrolled and randomly assigned to the experimental and control groups (n = 20 per group). Four participants subsequently withdrew, leaving 36 participants (n = 18 per group) in the final analysis. At pretest, participants completed the Ruminative Response Scale (Nolen-Hoeksema & Morrow, 1991), the Cognitive Inflexibility Scale (Dennis & Vander Wal, 2010), the External Shame Scale (Goss et al., 1994), and the Negative Affect subscale of the Positive and Negative Affect Schedule (Watson et al., 1988). The experimental group received emotion regulation-based therapy based on Gross’s (2002) model in eight 90-minute group sessions, whereas the control group received no intervention. Posttest data were analyzed using multivariate analysis of covariance (MANCOVA) in SPSS. The results indicated that the intervention significantly reduced rumination, cognitive inflexibility, external shame, and negative affect compared with the control condition, F(4, 27) = 21.46, p < 0.001, ηp² = 0.76.