<b><i>Background:</i></b> Vitamin D is a hormone regulating not only calcium and phosphate homeostasis but also, at the same time, exerting many other extraskeletal functions via genomic effects (gene transcription) and probably by non-genomic effects as well. Availability is ensured by dietary intake of its precursors and by de novo production via sunlight. Yet, vitamin D deficiency and insufficiency are very common across the globe and are connected to many pathophysiological states, for example, diabetes mellitus, allergies, autoimmune diseases, pregnancy complications, and recently have also been associated with worse COVID-19 clinical outcomes. <b><i>Summary:</i></b> In this review, we summarize current knowledge about vitamin D metabolism in general, its role in diabetes mellitus (mainly type 2) and diabetic complications (mainly diabetic kidney disease), and potential therapeutic perspectives including vitamin D signalling as a druggable target. <b><i>Key Messages:</i></b> Vitamin D is not only a vitamin but also a hormone involved in many physiological processes. Its insufficiency or deficiency can lead to many pathological states.