Background: Depression and anxiety disorders are considered to be among the greatest burdens of disease in children and adolescents. Recent literature has reported music therapy to be a safe and potentially effective intervention for the treatment of depression and anxiety. This systematic review examined the effectiveness of music therapy in reducing the symptom severity of depression and/or anxiety among children and adolescents. Methods: Randomized controlled trials (RCTs) were obtained from a systematic search of nine major English databases from inception to January 2021. Studies were categorized by outcome, music therapy technique and follow-up period. Results: Seven RCTs (n = 589) were included. Four RCTs had some concern of bias, while three had high risk of bias. Statistical pooling was not appropriate due to clinical heterogeneity. Four studies (n = 428) favoured music therapy for improvement of depressive symptoms at short-and intermediate-term follow-ups compared to control. One study (n = 106) favoured music therapy for improvement of anxiety symptoms at short-term follow-up compared to control. No studies favoured any control over music therapy for symptom improvement of depression and/or anxiety. Conclusion: Limited evidence suggests music therapy is an effective treatment for improving depression and/or anxiety symptom severity in children and adolescents. More high-quality RCTs are needed to address methodological flaws of current studies. Key Practitioner Message• There is evidence that suggests music therapy is an effective intervention in improving the symptoms of depression and/or anxiety in children and adolescents.• Previous literature did not address the clinical heterogeneity between the included studies; types of music therapy interventions, various controls and follow-up periods.• This review addressed the previous limitations by updating the search strategy and including more databases, stratifying outcomes based on the type of music therapy, assessing risk of bias and addressing the clinical heterogeneity between the included studies.• There is limited evidence that active and receptive music therapy techniques are effective in reducing depression symptoms among children and adolescents.• Further high-quality RCTs that include follow-up periods and stratification based on type of music therapy intervention are necessary to provide more robust conclusions regarding its effectiveness in improving symptoms of depression and/or anxiety in children and adolescents.