Purpose Recent observational studies have associated the use of androgen deprivation therapy (ADT) with an increased risk of dementia and Alzheimer's disease, but these studies had limitations. The objective of this study was to determine whether the use of ADT is associated with an increased risk of dementia, including Alzheimer's disease, in patients with prostate cancer. Patients and Methods Using the United Kingdom's Clinical Practice Research Datalink, we assembled a cohort of 30,903 men newly diagnosed with nonmetastatic prostate cancer between April 1, 1988 and April 30, 2015, and observed them until April 30, 2016. Time-dependent Cox proportional hazards models were used to estimate adjusted hazard ratios with 95% CIs of dementia associated with the use of ADT compared with nonuse. ADT exposure was lagged by 1 year to account for delays associated with the diagnosis of dementia and to minimize reverse causality. Secondary analyses assessed whether the risk varied with cumulative duration of use and by ADT type. Results During a mean (standard deviation) follow-up of 4.3 (3.6) years, 799 patients were newly diagnosed with dementia (incidence, 6.0; 95% CI, 5.6 to 6.4) per 1,000 person-years. Compared with nonuse, ADT use was not associated with an increased risk of dementia (incidence, 7.4 v 4.4 per 1,000 person-years, respectively; adjusted hazard ratio, 1.02; 95% CI, 0.87 to 1.19). In secondary analyses, cumulative duration of use ( P for heterogeneity = .78) and no single type of ADT were associated with an increased risk of dementia. Conclusion In this population-based study, the use of ADT was not associated with an increased risk of dementia. Additional studies in different settings are needed to confirm these findings.