There is increasing support for the efficacy of transdiagnostic cognitive behavioural interventions for anxiety and depression. However, little is known about the applicability of transdiagnostic behavioural interventions for children younger than 12 years old. This study was conducted to examine the feasibility and potential efficacy of Streamlined Transdiagnostic Intervention for Anxiety and Depression (STREAM) for children with anxiety and/or depressive disorders using a randomised controlled design with a wait-list control (WLC) condition and blind-assessments. Of the 22 potential participants, 16 Japanese children (M = 9.81; SD = 0.75; range 9–12 years) with principal anxiety or depressive disorder were eligible and enrolled. Then, the participants were randomly assigned to the STREAM or WLC condition. The dropout rates were 0% for both the conditions at post-assessment. Mixed model analyses showed that, although there were no significant interactions at post-assessment between both the conditions, both anxiety and depressive disorders significantly improved at 3 months compared with pre-assessment for the combined condition (the STREAM and WLC conditions). Therefore, this study demonstrated the feasibility of the STREAM in the Japanese clinical setting and potentially supported its efficacy for children with anxiety and depressive disorders at the follow-up assessment.