This study aimed to comprehensively analyze the clinical characteristics and prognosis of patients with concomitant bladder cancer (BCa) and prostate cancer (PCa) using a large population-based database.Within the Surveillance, Epidemiology, and End Results (SEER) database (2000-2019), we identified patient with concomitant PCa at the time of radical cystoprostatectomy (RCP). Logistic regression and propensity score matching (PSM) analyses were employed to identify risk factors and mitigate confounders, respectively. Kaplan-Meier survival curves were used to estimate cancer-specific survival (CSS).A total of 14,199 BCa patients undergoing RCP were identified, with 28.8% incidentally discovered to have concurrent PCa. Among them, 89.9% exhibited organ-confined (T1-2) PCa. An increased risk of concomitant tumors was observed among older age, white race, and high tumor grade of BCa. Survival analysis revealed no significant difference in CSS between patients with BCa alone and those with concurrent PCa (5-year CSS rate: 71.3% vs. 67.2%, P=0.076). Subgroup analysis and multivariable analysis, however, indicated that concurrent high-risk PCa adversely impacted survival (5-year CSS rate: 71.3% vs. 63.4%, HR 1.27, 95% CI 1.01-1.58, P=0.038) compared to solitary BCa. Notably, the presence of low/intermediate-risk PCa did not affect survival outcomes (P=0.584).In conclusion, incidentally discovered PCa in RCP specimens is frequent and characterized by organ-confined presentation, lower PSA levels, and Gleason scores. Patients with concurrent high-risk PCa have a worse prognosis compared to those with solitary BCa, while the presence of low/intermediate-risk PCa does not influence oncological prognosis.Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.