Anxiety and depression are debilitating and commonly co-occurring in young adolescents, yet few interventions are designed to treat both disorder classes together. Initial efficacy is presented of a school-based transdiagnostic group behavioral activation therapy (GBAT) that emphasizes anti-avoidance in vivo exposure. Youth (N = 35; ages 12–14; 50.9% male) were randomly assigned to either GBAT (n = 21) or WL (n = 14) after completing a double-gated screening process. Multi-reporter, multi-domain outcomes were assessed at pretreatment, posttreatment, and four-month follow-up (FU). GBAT was associated with greater posttreatment remission rates than WL in principal diagnosis (57.1% vs. 28.6%; = 2.76, p = .09) and secondary diagnosis (70.6% vs. 10%; = 9.26, p = .003), and greater improvement in Clinical Global Impairment – Severity ratings, B = −1.10 (0.42), p = .01. Symptom outcomes were not significantly different at posttreatment. GBAT produced greater posttreatment behavioral activation (large effect size) and fewer negative thoughts (medium effect), two transdiagnostic processes, both at the trend level. Most outcomes showed linear improvement from pretreatment to FU that did not differ depending on initial condition assignment. Sample size was small, but GBAT is a promising transdiagnostic intervention for youth anxiety and unipolar mood disorders that can feasibly and acceptably be applied in school settings.