Diabetes mellitus and periodontal disease are among the most prevalent chronic diseases worldwide and share a complex bidirectional relationship. Mounting evidence indicates that diabetes increases the risk, severity, and progression of periodontal disease, while periodontal inflammation adversely affects glycemic control and diabetic complications. Hyperglycemia contributes to periodontal tissue destruction through immune dysregulation, oxidative stress, microvascular dysfunction, and the formation of advanced glycation end products (AGEs). Conversely, periodontal disease promotes systemic inflammation through elevated cytokine production, thereby increasing insulin resistance and worsening metabolic control. This review comprehensively examines the biological mechanisms linking diabetes and periodontal disease, epidemiological evidence, clinical manifestations, diagnostic considerations, treatment outcomes, and future directions. Current research highlights the importance of integrated medical-dental management to improve both periodontal and metabolic outcomes. Advances in precision medicine, microbiome research, biomarker discovery, and digital healthcare technologies are expected to further strengthen interdisciplinary management strategies.