Cardiac surgery-associated acute kidney injury (CSA-AKI) is a critical complication associated with mortality and morbidity. This study aimed to clarify the impact of CSA-AKI on activities of daily living (ADL) at discharge in elderly cardiac surgery patients. We included 122 cardiac patients who underwent coronary artery bypass surgery, valve surgery, or combined surgery by mid-line incision followed by postoperative cardiac rehabilitation (CR) from March 2015 to May 2020. CSA-AKI was based on KDIGO criteria. The index of ADL was the Functional Independence Measure (FIM). We compared background factors, clinical parameters, activity level before hospitalization, CR progress, and FIM in patients with or without CSA-AKI. Multiple regression analysis was performed with FIM at discharge as the dependent variable and items with p 0.01 in bivariate correlation as independent variables. Ultimately, 122 patients were divided into the non-CSA-AKI group (n = 84) and CSA-AKI group (n = 38). CR progression in the CSA-AKI group was significantly slower and FIM was lower than that in the non-CSA-AKI group. Moreover, even after adjustment for confounding factors, CSA-AKI (\beta = –0.18), start day of walking (\beta = –0.34), postoperative atrial fibrillation (\beta = –0.15), and activity level before hospitalization (\beta = –0.37) were predictive factors of ADL decline at discharge (adjusted R{}^2 = 0.52). CSA-AKI of elderly cardiac surgery patients was a predictive factor of ADL decline at discharge.