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Diabetes mellitus (DM) is a group of metabolic diseases characterized by chronic hyperglycemia resulting from defects in insulin secretion, insulin action, or both. The most common types-type 1 and type 2 diabetes-have different etiologies and pathophysiological mechanisms. Type 1 diabetes (T1DM) results from autoimmune destruction of the insulin-producing pancreatic β-cells, leading to the development of absolute insulin deficiency, whereas in type 2 diabetes (T2DM), impaired carbohydrate metabolism is primarily caused by insulin resistance and relative insulin deficiency. Current diagnostic criteria do not allow for the detection of the disease at the preclinical stage. MicroRNA (miRNA) influences post-translational regulation of gene expression by inhibiting mRNA translation and also promotes mRNA degradation. The aim of this review is to summarize current evidence on the role of microRNAs in the pathogenesis of T1DM and T2DM and to evaluate their potential as early diagnostic biomarkers and therapeutic targets. It is demonstrated that T1DM and T2DM exhibit altered expression of specific microRNAs involved in β-cell apoptosis, autoimmune inflammation, and insulin signaling. In T1DM, key miRNAs include miR-21, miR-25, miR-146a, and miR-375, which reflect β-cell destruction and the autoimmune process. In T2DM, critical roles are played by miR-9, miR-29, miR-34a, miR-103/107, miR-126, miR-143, and miR-375, which regulate insulin secretion, lipid metabolism, and tissue insulin sensitivity. Particular attention is given to microRNAs whose expression changes several years before clinical disease onset (miR-15a, miR-126, miR-375), offering opportunities for early diagnosis. Data are presented on circulating miRNAs in stable biological fluids (blood, urine). It should be emphasized, however, that the proposed microRNA panel currently represents only a potential diagnostic tool. This panel requires further validation and confirmation by clinicians in large-scale prospective studies and does not yet claim to be ready for routine clinical use. Nevertheless, the development of such a universal microRNA panel, followed by thorough clinical evaluation, has promising biomedical potential, which will not only allow for the diagnosis of diabetes at an early stage but also identify new therapeutic targets for personalized medicine.