Introduction: The present study aimed to determine the effectiveness of the Unified Transdiagnostic Treatment on depressive symptoms and alexithymia in adolescents with self-injurious behaviors. Method: This research employed a quasi-experimental design with a pretest-posttest control group. The population consisted of all female students in the first grade of secondary schools in District 3 of Tehran during the 2023-2024 academic year (approximately 14,500 individuals). A purposive sample of 30 students was selected and randomly assigned into experimental and control groups (15 participants each). Research instruments included the Kutcher Adolescent Depression Scale (KADS-II), the Toronto Alexithymia Scale (TAS-20), and the Inventory of Statements About Self-Injury (ISAS). The experimental group received 12 sessions of Unified Transdiagnostic Treatment based on Barlow et al. (2011), while the control group received no intervention. Data were analyzed using multivariate and univariate ANCOVA. Findings: After controlling for pretest effects, there were significant differences between the experimental and control groups in depressive symptoms, alexithymia, and self-injurious behaviors (both intrapersonal and interpersonal) among adolescents (p < 0.01). Conclusion: Unified Transdiagnostic Treatment can be an effective intervention for reducing depressive symptoms, alexithymia, and self-injurious behaviors in adolescents and may be utilized by clinicians and mental health professionals in psychological centers for individuals exhibiting self-injurious behaviors.