STUDY OBJECTIVE: Acute kidney injury (AKI) is a common and serious complication after coronary artery bypass grafting (CABG) surgery. Patients with diabetes are commonly associated with renal microvascular complications and have a greater risk of AKI after CABG surgery. This study aimed to explore whether preoperative metformin administration could reduce the incidence of postoperative AKI following CABG in patients with type 2 diabetes. DESIGN: Patients with diabetes who underwent CABG were retrospectively included in this study. AKI after CABG was defined according to the Kidney Disease Improving Global Outcomes (KDIGO) criteria. The effects of metformin on postoperative AKI following CABG in patients were compared and analyzed. DATA SOURCE: Patients were enrolled in this study between January 2019 and December 2020 in Beijing Anzhen Hospital. PATIENTS: A total of 812 patients were enrolled. The patients were divided into the metformin group (203 cases) and the control group (609 cases) according to whether metformin was used preoperatively. INTERVENTION: Inverse probability of treatment weighting (IPTW) was applied to minimize baseline differences between the two groups. IPT-weighted p values were analyzed to evaluate the postoperative outcomes between the two groups. MEASUREMENTS AND MAIN RESULTS: The incidence of AKI in the metformin group and the control group was compared. After IPTW adjustment, the incidence of AKI in the metformin group was lower than the control group (IPTW-adjusted p < 0.001). In the subgroup analysis, metformin showed significant protective effects in the estimated glomerular filtration rate (eGFR) < 60 mL/min per 1.73 m2 and eGFR 60-90 mL/min per 1.73 m2 subgroups, which was not observed in the eGFR ≥90 mL/min per 1.73 m2 subgroup. No significant differences in the incidence of renal replacement therapy, reoperation due to bleeding, in-hospital mortality, or red blood cell transfusion volume were observed between the two groups. CONCLUSIONS: In this study, we provided evidence that preoperative metformin was associated with a significant reduction of postoperative AKI following CABG in patients with diabetes. Metformin showed significant protective effects in patients with mild-to-moderate renal insufficiency.