Background: Some studies suggest the existence of an “obesity paradox” in the intensive care unit (ICU). However, reports relating renal function and body mass index (BMI) are limited and controversial. This study aimed to examine the influence of BMI on acute kidney injury (AKI) in critically ill patients. Methods: We retrospectively analyzed the prospectively collected data from patients admitted to the ICU at Gyeongsang National University Hospital in Korea from December 2011 to May 2014. Patients were stratified to three classes according to their BMI (“underweight” ＜18.5 kg/m2, “normal” 18.5-24.9 kg/m2, and “overweight” =25 kg/m2). The occurrence of AKI was defined by Risk, Injury, Failure, Loss of kidney function, and End-stage kidney disease criteria, and the impact of BMI on AKI was analyzed. Results: A total 468 patients in the ICU were analyzed, and AKI occurred in 82 (17.5%) patients. The mean BMI was 21.5 ± 4.0 kg/m2; 102 patients (21.8%) were classified as “underweight,” 286 patients (61.1%) were classified as “normal,” and 80 patients (17.1%) were classified as “overweight.” The adjusted analysis showed significantly higher risk of AKI in the overweight group compared to the underweight group (odds ratio, 3.95; 95% confidence interval, 1.74-9). Additional risk factors for AKI in critically ill patients included comorbid liver cirrhosis, septic shock, and acute respiratory distress syndrome. Conclusions: BMI is a possible predictor of AKI in ICU patients, as this study indicated that AKI occurs more frequently in overweight patients than in underweight patients.