There is a lack of evidence for treatment of pediatric multiple sclerosis (PedMS). Treatment using azathioprine for PedMS has not been reported. A 10-year-old boy with multiple sclerosis who was seropositive for antibodies against myelin oligodendrocyte glycoprotein (MOG)-IgG was treated with azathioprine plus oral methylprednisolone. The patient showed clinical and magnetic resonance imaging stability, with MOG-IgG seroconversion. There were no major side effects over a 5-year period. Azathioprine may be a treatment option, particularly in poor medical resource areas, for pediatric patients with multiple sclerosis who are seropositive for MOG-IgG.