No study was conducted on the long-term risk of Alzheimer's disease (AD) and related dementias (ADRD) in association with vascular diseases in men with prostate cancer. To determine the 26-year risk of ADRD in association with cardiovascular disease (CVD), stroke, hypertension, and diabetes in a nationwide cohort of men with prostate cancer. Retrospective cohort study. Surveillance, Epidemiology, and End Results (SEER) areas of the United States. 351,571 men diagnosed with prostate cancer at age ≥65 years. Main exposures were CVD, stroke, hypertension, and diabetes. Main outcome was the incidence of ADRD. The crude 26-year cumulative incidence of any ADRD was higher in those with versus without CVD (33.80% vs 29.11%), stroke (40.70% vs 28.03%), hypertension (30.88% vs 27.31%), and diabetes (32.23% vs 28.68%). Men with CVD (adjusted hazard ratio: 1.17, 95% CI: 1.15-1.20), stroke (1.59, 1.56-1.61), hypertension (1.13, 1.11-1.14), and diabetes (1.25, 1.23-1.27) were significantly more likely to develop ADRD than those without. Patients with 4 of these vascular diseases were 161% more likely to develop ADRD (2.61, 2.47-2.76) than those without. The risk of AD (0.89, 0.87-0.91) and ADRD (0.91, 0.90-0.93) became significantly lower in men with prostate cancer who received androgen deprivation therapy as compared to those who did not after considering death as a competing risk. In men with prostate cancer, vascular diseases were associated with significantly higher risks of developing ADRD. Androgen deprivation therapy was associated with a significantly decreased risk of AD in men with prostate cancer.