Chronic kidney disease is common in older adults, and diabetes is a major contributor to kidney function decline. However, the relationship between glycemic control and long-term kidney function trajectories in older populations remains incompletely understood. We examined the association between diabetes, glycemic control, and longitudinal changes in kidney function using data from 9,281 participants in the Health and Retirement Study. Estimated glomerular filtration rate based on cystatin C (eGFR _cys ) was analyzed using linear mixed-effects models. Participants were categorized as having no diabetes, diabetes with good glycemic control (HbA1c < 7%), or diabetes with poor glycemic control (HbA1c ≥7%). Over 8 years, eGFR _cys declined by -6.50 mL/min/1.73 m² per 4-year interval among participants without diabetes. Individuals with diabetes experienced a faster decline compared with those without diabetes. When stratified by glycemic control, the rate of eGFR _cys decline did not differ significantly between participants with good glycemic control and those without diabetes, whereas those with poor glycemic control showed a substantially steeper decline (additional -3.98 mL/min/1.73 m² per 4-year interval). These findings suggest that poor glycemic control is associated with accelerated kidney function decline in older adults, whereas well-controlled diabetes shows trajectories similar to those without diabetes.