OBJECTIVE: Autism spectrum disorders (ASD) are neurodevelopmental disorders of early childhood which are characterized by limited social-emotional reciprocity and restricted, repetitive patterns of interest and behaviors. The pathophysiology of ASD has not been fully elucidated. Dysregulation of circadian rhythm and melatonin might play a role in ASD pathogenesis. This article aims to review the relationship between melatonin, circadian rhythm and ASD in detail. METHOD: Articles published in the PubMed database between 1990- 2019 were reviewed and prominent studies in the scope of the review were included. RESULTS: Decreased melatonin levels were detected frequently in ASD and melatonin replacement treatment for sleep disorders accompanying ASD has given satisfactory results. Similarly, circadian rhythm disorders were frequently reported in ASD, which might increase the susceptibility to ASD through their effects on synaptic plasticity in the early neurodevelopment. Mutations in the clock-controlled genes were also common in ASD. CONCLUSION: Further are required for understanding the relationship between melatonin, circadian rhythm and ASD, which will not only shed light on the role of melatonin in the etiology of ASD, but may also guide to early intervention options.