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Vitamin D3 has long been known to affect the skeletal system and mineral metabolism. In recent years, there has been an increasing interest in the effects of vitamin D3 on different systems. The relation of vitamin D3 with brain development and autism is an intensely researched area in this context. The aim of this paper is to review the current evidence on the effects of vitamin D3 on brain development and autism and to provide insights for future scientific studies. Vitamin D3 has been shown to have direct effects on neural development, neural cell proliferation and apoptosis. Moreover, this vitamin also has effects on the immune system, inflammation processes, and antioxidation, which in turn are known to have effects on brain development. Current evidence suggests that vitamin D3 has a critical effect on brain development and that vitamin D3 deficiency might have detrimental effects on mental development. Similarly, vitamin D3 deficiency and autism show many similarities in their etiopathogenesis. However, clinical studies showing the link between vitamin D3, brain development and autism are limited. The data obtained in this area are based on animal studies, and current data do not seem to be sufficient to allow for direct conclusions.