Background: This study investigated the independent and joint impacts of the geriatric nutritional risk index (GNRI) and frailty index (FI) on all-cause and cause-specific mortality in patients with diabetes. Methods: This study included 1,790 National Health and Nutrition Examination Survey participants between 1999 and 2018. GNRI and FI were used to assess patients with diabetes, who were categorized into low GNRI and high GNRI groups, as well as low FI and high FI groups, to evaluate older adults’ nutritional status. These groups were combined into four categories. Weighted Cox proportional hazards models, Kaplan-Meier survival curves, and restricted cubic spline regression models were applied to assess the independent and joint associations of GNRI and FI with all-cause and cause-specific mortality in patients with diabetes. Results: The median follow-up period was 100 months, during which 720 deaths occurred. An independent linear dose-response relationship between GNRI, FI, and all-cause mortality risk was observed. The combination of low GNRI and high FI was significantly associated with increased risk of all-cause mortality (hazard ratio, 1.65; 95% confidence interval [CI], 1.02 to 2.68) for the low GNRI+low FI group, 2.03 (95% CI, 1.61 to 2.55) for the high GNRI+high FI group, and 3.65 (95% CI, 2.38 to 5.58) for the low GNRI+high FI group. Stratified analysis (reference: high GNRI+low FI group) showed an increased mortality risk among men in the low GNRI+high FI group. Conclusion The combination of low GNRI and high FI was independently and jointly associated with increased risk of all-cause, cardiovascular disease, diabetes, and hypertension mortality, particularly among men.