As a fat-soluble steroid hormone, vitamin D is crucial not only for bone health but also may influence the occurrence and development of diabetes mellitus and its complications. Studies have shown that vitamin D deficiency may be involved in insulin resistance and pancreatic β-cell dysfunction, leading to the development of type 1 and type 2 diabetes. Vitamin D supplementation may help prevent type 1 diabetes and improve blood glucose control in patients with type 2 diabetes. This article reviews the research progress on vitamin D and diabetes mellitus and its related complications.