Preoperative metformin use in patients with type 2 diabetes remains controversial due to concerns about lactic acidosis, despite inconsistent evidence. Current guidelines provide no clear consensus on whether to continue or hold metformin prior to surgery, leaving clinicians to make individualized decisions. This study assessed the association between preoperative metformin use and postoperative complications, including infections (pneumonia, urinary tract infection, surgical site infection, sepsis, or septic shock) and acute kidney injury (AKI). We used electronic health records from 13 hospitals standardized to the OMOP-CDM to include adults (≥19 years) with type 2 diabetes who underwent surgery. Metformin users were defined as those who had at least two prescriptions with a 30-day drug era within the year before surgery and no history of dialysis. Comparator group included those with no prescription for metformin during the same period. The primary outcome was postoperative infection within 90 days, supplemented by C-reactive protein (CRP) and procalcitonin (PCT) levels. The safety outcome was AKI, assessed using eGFR and creatinine levels. We applied 1:1 propensity score matching and Cox proportional hazards model, pooling results via fixed-effect meta-analysis. Among 36,946 patients, preoperative metformin use was associated with a lower risk of postoperative infection (HR 0.85; 95% CI 0.76–0.95), with a more pronounced benefit in patients with CRP levels of 50–100 mg/L (HR 0.80; 95% CI 0.72–0.91). However, metformin use was also associated with an increased risk of AKI (HR 1.43; 95% CI 1.10–1.86). These findings highlight the need for individualized risk assessment when considering perioperative metformin use. Balancing between the reduced risk of postoperative infections and the increased risk of AKI is essential in guiding clinical decisions.