This study was designed to determine the protective effect of kefir on the oxidative damage, as well as histological and biochemical changes that occur in the kidney tissues of experimental diabetic rats. Forty rats were allocated into four groups as "healthy saline" (SF), "healthy kefir" (KF), and the same groups with experimentally induced diabetes (DSF and DKF). Diabetes was induced by administering 65 mg/kg single-dose streptozotocin. Oral kefir was given 10 ml/kg/day for 35 days to the rats in the groups KF and DKF. The same amount of saline was given to the groups SF and DSF. On the 36th day of the study, blood glucose, urea, and creatinine were measured besides glucose, creatinine, microalbuminuria, and sodium in the urine. Additionally, histological examination was performed on the kidney tissues. Blood glucose, creatinine, and urea levels were significantly lower in the DKF group compared with those of the DSF group (p < 0.001). Also, the creatinine level was significantly decreased (p < 0.001), and microalbuminuria was increased (p < 0.001) in the DKF group compared with that of the DSF group. Histologically, intermittent expansion in the renal glomeruli, reduction of cast formation in the tubules, and improvement in the renal epithelial tissues of the DKF group were observed. Kefir decreased the damage caused by diabetes. These results indicate that kefir supplementation may contribute to better control of oxidative stress, which is related to the improvement of renal functions, suggesting its use to slow down the progression of diabetic nephropathy.