Background Acute kidney injury (AKI) after cardiopulmonary bypass (CPB) is associated with a requirement for dialysis, a longer stay in the intensive care unit, a longer hospital length of stay, and mortality. An oxygenator arterial outlet temperature greater than 37°C has been reported to be associated with AKI; however, the influence of other rewarming temperatures is unclear. Using multicenter registry data, this study aimed to evaluate the role of CPB rewarming temperatures on AKI. Methods Data from 8,407 adult patients undergoing coronary artery bypass grafting (CABG) or valve repair or replacement, or a combination, were collected using the Perfusion Downunder Collaborative Database. Primary variables of interest were rewarming temperatures, defined as cumulative time the oxygenator arterial outlet temperature was greater than 36°C, greater than 36.5°C, or greater than 37°C. Propensity scores were calculated to determine the predicted probability of hyperthermic perfusion (rewarming temperature >37°C). The influence of temperature on AKI was determined using separate multivariate models adjusting for propensity score in the entire cohort (n = 6,904) and in propensity-matched patients (n = 2,044). Results Overall, 11.8% of patients acquired AKI. The duration of rewarming temperature greater than 36°C or 36.5°C was not associated with AKI. The duration of rewarming temperature greater than 37°C (hyperthermic perfusion) was independently associated with RIFLE (Risk, Injury, Failure, Loss, End-stage renal disease) risk classification or greater (odds ratio [OR], 1.42; 95% confidence interval [CI], 1.09-1.77; p = 0.012) and injury classification or greater AKI (OR, 1.52; 95% CI, 1.09-1.97; p = 0.016) in the entire cohort, and injury classification or greater AKI (OR, 1.51; 95% CI, 1.15-1.90; p = 0.006) in propensity-matched patients. Conclusions The duration of hyperthermic perfusion - rewarming temperature greater than 37°C - was an independent predictor of AKI. Avoidance of hyperthermic perfusion may be more beneficial in reducing AKI than avoidance of rewarming.