Purpose <p>This study aimed to assess type 2 diabetes mellitus (T2DM)-related avoidable mortality in individuals with developmental disabilities using national claims data, comparing outcomes with those of other disabilities and non-disabled controls.</p> Methods <p>We used de-identified national claims data from the National Health Insurance Service merged with Causes of Death Statistics. The cohort included individuals over 30 years old with developmental disabilities who were newly diagnosed with T2DM between 2012 and 2016. Matched controls (non-disabled, mild, and severe disabilities) were selected based on sex, age, income proxy, and region. T2DM-related avoidable mortality (ICD-10 code “E11”) within 1, 3, and 5 years was assessed using multiple logistic regression, adjusted for sociodemographic factors and comorbidities.</p> Results <p>T2DM-related avoidable mortality rates in the developmental disability group were 0.05%, 0.23%, and 0.55% at 1, 3, and 5 years post-diagnosis, respectively—lower than those in individuals with severe disabilities but higher than those of individuals without disabilities. Multiple logistic regression revealed no significant difference in 1-year mortality between individuals with developmental disabilities and those without disabilities; however, the risk was significantly higher at 3 years (OR = 4.84; 95% CI: 1.80–13.00) and 5 years (OR = 3.82; 95% CI: 2.14–6.81). Compared with individuals with mild disabilities, the 5-year mortality risk was also higher (OR = 2.41; 95% CI: 1.38–4.21).</p> Conclusion <p>Individuals with developmental disabilities exhibit significantly higher T2DM-related avoidable mortality than non-disabled and mild disability groups, highlighting critical gaps in healthcare accessibility. Strengthening targeted interventions and support services is essential to reducing avoidable deaths and improving health outcomes in this population.</p>