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.