Gestational diabetes mellitus (GDM), defined as any glucose intolerance with the onset or first recognition during pregnancy, is characterized by rising incidence, fostered by the worldwide increase of pathological nutritional status from young age. Clinical research has intended to identify potential risk factors, suggested improvements in screening strategies, and recommended the combination between promotion of an appropriate lifestyle before and during pregnancy and selected therapeutic approaches. Preventing pathological hyperglycemia could have several benefits, ranging from clinical side (reduction in the risk of adverse perinatal and long-term sequelae) to financial side (cost reduction to healthcare systems). Among risk factors recognized, deficiency in 25-hydroxyvitamin D [25(OH)D], already acknowledged as involved in calcium homeostasis, pathogenesis of cardiovascular, oncological, infective and immunity diseases, could predispose to the development of both type 1 and 2 diabetes, modifying the activity of pancreatic β-cells vitamin D (VD) receptor. In pregnant women, lower 25(OH)D concentrations have been suggested to present an inverse association with maternal glycaemia, insulin resistance, and increased risk of GDM. In spite of growing body of evidence, there is not full agreement on the therapeutic association between GDM based on VD deficiency and 25(OH)D supplementation. In the attempt to bring up-to-date the role of low VD levels on subsequent development of GDM, this narrative review, based on medium-high-quality randomized clinical trials, systematic reviews, and meta-analysis published in last decade, has a twofold purpose: firstly, to elucidate the relationship between maternal VD status and GDM; and secondly, to illuminate the impact of VD supplementation on GDM onset.