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BACKGROUND Thyroid cancer (TC) and prostate cancer (PC) are common endocrine-related malignancies. Given the growing population of patients diagnosed with both cancers, their prognosis requires clarification. This study aimed to compare mortality between patients with both TC and PC and those with TC or PC alone. MATERIAL AND METHODS In this retrospective cohort study, data were derived from the Surveillance, Epidemiology, and End Results (SEER) database (2000-2021), including 2334 men with records of both TC and PC (TC+PC group), 37 979 men with only TC (TC-only group), and 876 838 men with only PC (PC-only group). Cox proportional hazards regression and competing risk models were used to analyze patient mortality, supplemented by a propensity score-matching sensitivity analysis. RESULTS Cox regression modeling showed that survival among patients with TC and a history of PC was better than among patients with TC and no history of PC. Hazard ratios (HRs) for all-cause mortality and TC-specific mortality, compared with the TC-only group, were 0.78 (95% confidence interval [CI], 0.71-0.87) and 0.58 (95% CI, 0.47-0.72), respectively. Similarly, patients with PC and a history of TC showed reduced all-cause mortality (HR=0.83, 95% CI, 0.75-0.91) and PC-specific mortality (HR=0.62, 95% CI, 0.50-0.78). Superior survival in the TC+PC group was confirmed by competing risk models and propensity score-matching analysis. CONCLUSIONS This study demonstrates a mutually favorable survival association between TC and PC, indicating that a history of either cancer warrants consideration in prognostic evaluation.