Diabetes mellitus and periodontitis share a strong bidirectional relationship, in which hyperglycemia increases periodontal inflammation and tissue destruction, while periodontal inflammation worsens insulin resistance and glycaemic control. Chronic hyperglycemia promotes oxidative stress, advanced glycation end-product (AGE) accumulation, immune dysregulation, and a shift toward pro-inflammatory pathways (M1 macrophages, Th17 cells), all of which accelerate periodontal breakdown. Conversely, periodontal infection elevates systemic inflammatory mediators such as IL-1β, IL-6, and TNF-α, contributing to poorer metabolic control in diabetic patients. Non-surgical periodontal therapy (scaling and root planing) can modestly improve HbA1c levels and reduce systemic inflammation, highlighting the clinical importance of integrated care.