The Unified Protocol for Transdiagnostic Treatment of Emotional Disorders in Children (UP-C) aims to reduce internalizing symptoms in children by addressing fundamental mechanisms that underlie emotional disorders through a combination of cognitive, emotional, and behavioral strategies. This study, the largest randomized controlled trial (RCT) to date of UP-C efficacy and the first one conducted outside the U.S., assesses the efficacy of UP-C compared to an active control group. A total of 153 children (6–13 years old; 58.8% girls) with at least one emotional disorder as a primary diagnosis and their parents (88.2% mothers) were randomly assigned to either the UP-C condition (n = 77) or an active control condition (n = 76; receiving a psychoeducational intervention, the ABC of Emotions). Participants completed self-report questionnaires at pretreatment, midtreatment, posttreatment, and at the 3-month follow-up, assessing children’s internalizing symptoms and their interference with the child and family functioning. Clinicians assessed the severity of the child’s symptoms and monitored clinical improvement throughout the study. Descriptive statistics, comparative tests and Linear Mixed Models were used. Results showed greater satisfaction, lower dropout rates, and better adherence in the UP-C condition, with a reduced need for additional interventions posttreatment compared to the control group. The UP-C demonstrated significantly greater efficacy in reducing internalizing symptoms, with a large effect size, including anxiety and depression, as well as in reducing the interference and severity of child symptoms and in improving clinical gains compared to the control group. These findings provide strong support for UP-C as a feasible, acceptable, and effective treatment for childhood emotional disorders, reinforcing its status as an evidence-based treatment.