Introduction <p>In recent years, Red Blood Cell Distribution Width (RDW) and serum albumin have been recognized as important markers of inflammation and nutritional status. The ratio of RDW to serum albumin (RAR) is considered a comprehensive indicator that combines both inflammatory and nutritional characteristics. However, the potential relationship between RAR and all-cause mortality as well as cardiovascular mortality in diabetic patients has not been fully explored.</p> Aim <p>To investigate whether RAR is associated with all-cause mortality and cardiovascular mortality in diabetic patients, and to examine the dose-response relationship between them.</p> Methods <p>This study used data from the NHANES collected between 1999 and 2018, including 4254 participants aged 18 years and older, with RDW and serum albumin levels obtained from laboratory tests. Weighted Cox regression and adjusted models were used to explore the relationship between RAR and specific mortality rates. Stratified analysis was performed to examine the relationship between mortality and specific subgroups, and restricted cubic splines (RCS) were used to explore the dose-response relationship between RAR and specific mortality. Statistical significance was set at a two-tailed <i>p</i>-value &lt; 0.05.</p> Results <p>Among diabetic patients, higher baseline RAR levels were significantly associated with increased risks of all-cause mortality, heart disease mortality, and cardiovascular disease mortality, even after adjusting for multiple confounding factors. However, no statistically significant association was found between RAR and cerebrovascular disease mortality. Kaplan-Meier survival analysis and RCS analysis revealed a significant dose-response relationship between RAR and all-cause mortality, cardiovascular disease mortality, and heart disease mortality. This relationship showed a nonlinear increasing trend once RAR reached a certain threshold, suggesting that RAR elevation does not simply linearly increase risk, but rather accelerates the pathophysiological imbalance, leading to a sharp increase in mortality risk. Stratified analysis indicated heterogeneity in the association between RAR and heart disease mortality across different subgroups. For example, in diabetic patients not using antidiabetic medications, the predictive value of RAR may be stronger. In addition, the association between RAR and heart disease mortality was stronger in male patients than in female patients. Furthermore, the association between RAR and all-cause mortality, heart disease mortality, and cardiovascular disease mortality was particularly significant in smokers, patients with hypertension, and those with cardiovascular disease.</p> Conclusions <p>This study found that in diabetic patients, an elevated RAR is significantly associated with increased risks of all-cause mortality, heart disease mortality, and cardiovascular disease mortality, suggesting that RAR could serve as a convenient biomarker for identifying high-risk individuals. However, no significant association was found between RAR and cerebrovascular disease mortality.</p>