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Dietary fiber is recommended in diabetes management for its potential anti-inflammatory effects, but currently, the association between fiber intake, systemic inflammation, and long-term survival in adults with diabetes remains uncertain. This cohort study analyzed data from 3894 U.S. adults with diabetes across 10 National Health and Nutrition Examination Survey cycles (1999-2020). Fiber intake levels (grams of fiber per kilogram of body weight, g/kg) was assessed via dietary interviews, and mortality outcomes were linked to National Death Index records through 2019. The systemic immune-inflammation index (SII) was calculated using platelet, neutrophil, and lymphocyte counts. Cox proportional hazards models evaluated associations between fiber intake and all-cause mortality, adjusted for demographics, lifestyle factors, and comorbidities. Mediation analysis quantified SII's role in mortality risk reduction. Higher fiber intake levels demonstrated a dose-dependent inverse relationship with all-cause mortality (P < .001). In fully adjusted models, the 3rd quartile (Q3) of fiber intake levels showed the lowest mortality risk (hazard ratio = 0.47, 95% confidence interval = 0.37-0.68). The association attenuated slightly in Q4, possibly indicating a threshold effect. Mediation analysis revealed that 13.07% (95% confidence interval = 6.85%-18%) of the mortality reduction was attributable to SII downregulation. Increased dietary fiber intake levels are associated with reduced all-cause mortality in diabetic populations, with a possible mediating factor being the reduction in systemic inflammation levels among U.S. adults with diabetes. These findings support integrating fiber-rich diets into diabetes management protocols.