Background <p>This multi-institute retrospective cohort study investigated the association between preoperative anemia and postoperative acute kidney injury (AKI) in patients undergoing metabolic and bariatric surgery (MBS).</p> Methods <p>Using the TriNetX research network, we identified adult patients who underwent MBS between January 2010 and December 2024. Patients were categorized as anemic or non-anemic based on preoperative hemoglobin levels (hemoglobin &lt; 12 g/dL for females and &lt; 13 g/dL for males). Propensity score matching was performed to balance the baseline characteristics. The primary outcome was AKI within 30 days of MBS. The secondary outcomes included infections, intensive care unit admission, surgical complications, and deep vein thrombosis. Analyses were conducted separately for female and male patients, with additional sensitivity analyses for the female subgroups.</p> Results <p>After propensity score matching, preoperative anemia was significantly associated with an increased risk of postoperative AKI in females (1.51% vs. 0.71%, odds ratio (OR) 2.14, 95% confidence interval (CI) 1.53–2.98, <i>p</i> &lt; 0.0001) (<i>n</i> = 7144 pairs). This association persisted in all sensitivity analyses. Even mild anemia (hemoglobin 10–12 g/dL) was associated with a higher AKI risk (OR 1.62, <i>p</i> = 0.004) in female patients. In males (<i>n</i> = 971 pairs), anemia similarly increased the AKI risk (4.94% vs. 2.68%, OR 1.89, 95% CI 1.16–3.07, <i>p</i> = 0.009). No significant differences were observed in secondary outcomes for either sex.</p> Conclusion <p>Preoperative anemia is independently associated with an increased risk of postoperative AKI in both female and male patients undergoing MBS, suggesting that preoperative anemia screening and management could be important for reducing postoperative kidney complications in this population.</p> Graphical Abstract <p></p>