The most common mental health problems among adolescents are anxiety and mood disorders. While disorder-specific cognitive behavior therapy (CBT) is effective for each of these conditions, the comorbidity between anxiety and mood disorders indicates a need for the development of evidence-based transdiagnostic treatments. To examine the efficacy of culturally adapted transdiagnostic CBT (CA-CBT) in reducing symptoms of anxiety and depression in treatment-resistant Turkish adolescents, 13 adolescent participants with anxiety or mood disorders who were treatment resistant received 10 sessions of CA-CBT in group format. The main outcome measures were the Screen for Childhood Child Anxiety Related Disorders (SCARED), Beck Depression Inventory (BDI), and the Turkish Symptom and Syndrome Addendum (TSSA), which were assessed at baseline, posttreatment, and at 2-month follow-up. At posttreatment, there were large effect sizes for all measures: depression scores (BDI, d = .9), anxiety scores (SCARED, d = 1.1), and the Turkish Symptom and Syndrome Addendum (TSSA, d = 1.6). Moreover, at 2-month follow-up, depression and anxiety symptoms were either maintained or continued to improve such that from pretreatment to follow-up the effect sizes were as follows: depression scores (BDI, d = 1.4), anxiety scores (SCARED, d = 1.7), and the Turkish Symptom and Syndrome Addendum (TSSA, d = 2.4). In addition, there were no dropouts across treatment. This open trial suggests that CA-CBT is effective in reducing anxiety and depression symptoms and that the treatment is well accepted. A full randomized controlled trial to verify the effectiveness of transdiagnostic CA-CBT in similar populations is needed. (PsycINFO Database Record (c) 2019 APA, all rights reserved).