BACKGROUND AND OBJECTIVES: Low serum 25-hydroxyvitamin D (25[OH]D) concentration has been associated with higher levels of proteinuria and lower levels of eGFR in observational studies. In the Vitamin D and Type 2 Diabetes (D2d) study, we investigated the effect of vitamin D supplementation on kidney outcomes in a population with prediabetes. DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS: Overweight/obese adults with high risk for type 2 diabetes (defined by meeting two of three glycemic criteria for prediabetes) were randomized to vitamin D3 4000 IU per day versus placebo. Median duration of treatment was 2.9 years (interquartile range 2.0-3.5 years). Kidney outcomes included (1) worsening in Kidney Disease: Improving Global Outcomes (KDIGO ) risk score (low, moderate, high, very high) on two consecutive follow-up visits after the baseline visit and (2) mean changes in eGFR and urine albumin-to-creatinine ratio (UACR). RESULTS: Among 2166 participants (mean age 60 years, body mass index 32 kg/m2, serum 25(OH)D 28 ng/ml, eGFR 87 ml/min per 1.73 m2, UACR 11 mg/g, 79% with hypertension), 10% had moderate, high, or very high KDIGO risk score. Over a median follow-up of 2.9 years, there were 28 cases of KDIGO worsening in the vitamin D group and 30 in the placebo group (hazard ratio, 0.89; 95% confidence interval [95% CI], 0.52 to 1.52]). Mean difference in eGFR from baseline was -1.0 ml/min per 1.73 m2 (95% CI, -1.3 to -0.7) in the vitamin D group and -0.1 ml/min per 1.73 m2 (95% CI, -0.4 to 0.2) in the placebo group; between-group difference was -1.0 ml/min per 1.73 m2 (95% CI, -1.4 to -0.6). Mean difference in UACR was 2.7 mg/g (95% CI, 1.2 to 4.3) in the vitamin D group and 2.0 (95% CI, 0.5 to 3.6) in the placebo group; between-group difference was 0.7 mg/g (95% CI, -1.5 to 2.9). CONCLUSIONS: Among persons with prediabetes, who were not preselected on the basis of serum 25(OH)D concentration, vitamin D supplementation did not affect progression of KDIGO risk scores and did not have a meaningful effect on change in UACR or eGFR.