To evaluate long-term renal function following radical cystectomy (RC) for bladder cancer and identify risk factors associated with postoperative decline in renal function. The study included patients who underwent RC at a single centre in Denmark between 2009 and 2019. Data was collected through national electronic medical records. Renal function was evaluated by estimated glomerular filtration rate (eGFR) using pre- and postoperative creatinine measurements. Cumulative incidence and Cox Proportional Hazards models were used to describe the loss of renal function and its association with clinicopathological variables, as well as its effect on mortality. After exclusions, 670 patients were eligible for analyses. Median follow-up time was 6.2 years (interquartile range 4.0 -8.4). The proportion of patients with renal insufficiency (eGFR<45 mL/min) increased from 8.9% before RC to 19% 5 years after surgery. A total of 610 patients with preoperative eGFR≥45 were included in survival analyses. The absolute risk of renal function decline to CKD stage G3b or worse (eGFR<45 mL/min) was 17% (95% CI 14 -20) at 5 years postoperatively. Loss of renal function was not significantly associated with higher all-cause mortality. In multivariate analysis lower preoperative eGFR, diabetes mellitus, prior pelvic radiation therapy, continent urinary diversion types, and postoperative ureteral stricture were all independently associated with renal function decline. The long-term renal function decreases considerably for a large number of RC patients. Recognizing preoperative risk factors could identify patients who benefit from enhanced renal surveillance or early intervention for modifiable factors to minimize renal insufficiency following RC.