A review of scientific publications from 2010–2025 (PubMed, eLibrary, WHO, IDF, ScienceDirect) was conducted. Clinical, epidemiological, and molecular studies were included, focusing on the relationship between 25(OH)D and insulin resistance, β-cell function, inflammation, as well as diabetic neuropathy, nephropathy, and cardiovascular complications. Vitamin D deficiency (25(OH)D < 50 nmol/L) occurs in 60–90% of the population and is closely associated with obesity, metabolic syndrome, and insulin resistance. Domestic studies (Karonova T. L. et al., 2015; Stepanova A. P. et al., 2018; Pigarova E. A., Dzeranova L. K., 2024) confirm an inverse relationship between 25(OH)D and glucose levels, HbA1c, HOMA-IR, and body weight. Global meta-analyses (Pittas A. G., 2019; Song Y., 2019 BMJ) demonstrate a reduced risk of T2DM with adequate vitamin D concentrations. Molecular data indicate the involvement of the vitamin D receptor (VDR) and the CYP27B1 enzyme in the regulation of β-cell insulin secretion, Ca²⁺ metabolism, and NF-κB inflammatory pathways. Vitamin D deficiency is a modifiable non-classical risk factor for T2DM and its complications. Screening and correction of 25(OH)D levels is recommended in individuals with obesity, insulin resistance, and prediabetes