Although postoperative acute kidney injury (AKI) is a serious complication after cardiac surgery, preventive measures are limited. Despite the known association of preoperative low magnesium levels with cardiac surgery–related atrial fibrillation, the association between preoperative magnesium concentration and postoperative AKI has not been fully elucidated. This study evaluated the association between preoperative serum magnesium level and the development of AKI after cardiac surgery.