Erectile dysfunction (ED) is a well-known predictor for future cardiovascular and cerebrovascular disease. However, the relationship between ED and dementia has rarely been examined. This study investigates the longitudinal risk for Alzheimer's disease and non-Alzheimer dementia in patients with ED. We collected a random sample of 1,000,000 individuals from Taiwan's National Health Insurance database. From this sample, we identified 4153 patients with newly diagnosed ED between 2000 and 2009 and compared them with a matched cohort of 20,765 patients without ED. All patients were tracked for 7 years from the index date to identify which of them subsequently developed dementia. During the 7-year follow-up period, the incidence rate of dementia in the ED cohort was 35.33 per 10,000 person-years. In the comparison groups, it was 21.67 per 10,000 person-years. After adjustment for patients characteristics and comorbidities, patients with ED were 1.68-times more likely to develop dementia than patients without ED (95% CI = 1.34-2.10, P < 0.0001). In addition, older patients and those with diabetes, hypertension, chronic kidney disease, stroke, depression, and anxiety were found to be at increased risk for dementia. Analyzing the data by dementia type, we found the hazard risk for Alzheimer's disease and non-Alzheimer dementia to be greater in patients with ED (adjusted HR 1.68, 95% CI = 1.31-2.16, P < 0.0001 and 1.63, 95% CI = 1.02-2.62, P = 0.0429, respectively). Log-rank test revealed that patients with ED had significantly higher cumulative incidence rates of dementia than those without (P < 0.0001). Patients with ED are at an increased risk for dementia later in life.