Background: Acute kidney injury (AKI) frequently occurs in hospitalized patients, particularly in the elderly. However, studies on outcome-modifying factors in geriatric patients with AKI are absent, especially the influence of body mass index (BMI). Methods: We performed a retrospective analysis of a prospectively collected multicenter observational cohort, which enrolled elderly (≥65 years) who developed AKI after major surgery in the intensive care units. We analyzed in-hospital mortality within BMI category utilizing Cox proportional hazard regression analysis and generalized additive modeling. Results: Data of a total of 2,015 postoperative elderly patients were retrieved and analyzed. Generalized additive modeling showed that elderly AKI patients with a BMI between 21 and 31 kg/m2 (“normal”) had a lower mortality risk than those with a BMI of less than 21 kg/m2 (“underweight”) or 31 kg/m2 or greater (“obese”). Both “underweight” and “obese” individuals had a greater risk of mortality compared with patients with “normal” BMI. Conclusions: The U-shaped association of BMI with hospital mortality in geriatric AKI patients contains a widened base and a shifted nadir comparing with chronic dialysis and other AKI patients. This finding is interesting and warrants our attention.