Introduction: A potassium-rich DASH diet combined with low sodium reduces blood pressure. However, the effects of sodium reduction in combination with the DASH diet on kidney function are unknown. We aimed to determine the effects of sodium reduction and the DASH diet, on estimated glomerular filtration rate (eGFR) by cystatin C. Methods: DASH-Sodium was a controlled, feeding study in adults with elevated or stage 1 hypertension, randomly assigned to the DASH or a control diet. On their assigned diet, participants consumed each of three sodium levels for 30 days following a 2-week run-in period of a high sodium-control diet. The three sodium levels were low (50 mmol/d), medium (100 mmol/d), and high (150 mmol/d). The primary outcome was change in eGFR based on cystatin C. Results: Cystatin C was measured in 409 of the original 412 participants of which 207 were assigned the DASH diet and 202 to the control diet. Compared with control, the DASH diet did not affect eGFR (β=-0.96 mL/min/1.73 m²; 95%CI: -2.74, 0.83). In contrast, low versus high sodium intake decreased eGFR (β=-2.36 mL/min/1.73 m²; 95%CI: -3.64, -1.07). Together, compared to the high sodium-control diet, the low sodium-DASH diet decreased eGFR by 3.10 mL/min/1.73 m² (95%CI: -5.46, -0.73). This effect was attenuated with adjustment for diastolic blood pressure and 24-hr urinary potassium excretion. Conclusions: A combined low sodium-DASH diet reduced eGFR over a 4-week period. Future research should focus on the impact of these dietary interventions on subclinical kidney injury and their long-term impact on progression to chronic kidney disease.