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Objective: To estimate the long-term risks of severe late toxicities for radiation therapy (RT) following radical prostatectomy (RP) in an unselected nationwide cohort, as severe side-effects are rare but may occur years later. Patients and Methods: The study population comprised all men undergoing RP between 1997 and 2016 in the Prostate Cancer database Sweden (PCBaSe) (n = 40 962). By (1:2) matching, two cohorts were created: 2789 men exposed to postoperative RT and 5578 unexposed men with comparable age, comorbidities, and year of surgery. Cumulative incidences and rate ratios were calculated for the following outcomes: symptoms and interventions of the urinary or intestinal tract demanding inpatient care, secondary malignancies, and non-prostate cancer mortality. Results: The largest differences were seen for late toxicities affecting the urinary tract. The 10-year cumulative incidences among those exposed to postoperative RT vs the RP-only group were: 17.8% vs 10.5% for procedures of the urinary tract (difference 7.3%, 95% confidence interval [CI] 4.4 to 10.3; relative risk [RR] 1.74, 95% CI 1.47 to 2.05); 6.0% vs 1.2% for haematuria (difference 4.8%, 95% CI 3.1 to 6.5; RR 6.50, 95% CI 4.31 to 10.10); and 2.4% vs 1.1% for bladder cancer (difference 1.4%, 95% CI 0.4 to 2.3; RR 2.71, 95% CI 1.72 to 4.33). The groups were similar regarding intestinal toxicity, other secondary malignancies, and non-prostate cancer mortality. Adjustments for preoperative tumour risk factors did not importantly affect the rate ratios. Conclusion: Severe late toxicity after postoperative RT following RP predominately affects the bladder and can appear many years after RT.