Objectives The Korean Medication Algorithm Project for Depressive Disorder 2021 (KMAP-DD 2021) was a revision of previous works. The main purpose of the current study was to amend guidelines for the treatment of a major depressive disorder (MDD) for children and adolescents. Methods The survey consisted of 21 questionnaires for children and adolescents. A total of 33 of the 46 experts in child and adolescent psychiatry answered the survey. Results Antidepressant (AD) monotherapy was selected as the 1st line option for MDD with mild to moderate severity. As the 1st line of treatment for MDD severe without psychotic features in children and adolescents, AD monotherapy and AD augmented with atypical antipsychotics (AAP) were recommended. For MDD with psychotic features, AD augmented with AAP was preferred as the 1st line of treatment. Conclusion We developed an algorithm for child and adolescent populations with depressive disorders, more specifically than the KMAP-DD 2017. We expect this algorithm will provide clinicians useful information and help in the treatment of children and adolescents with depressive disorders.