Osteoarthritis (OA) and dementia are prevalent causes of disability in geriatric patients. To date, information on the temporal correlation between these progressive diseases and the risk of dementia in patients with OA is limited. This retrospective population-based 4-year cohort study investigated the risk of dementia in patients with OA. We performed a case-control matched analysis by using the Taiwan Longitudinal Health Insurance Database 2005. Patients were selected on the basis of International Classification of Diseases, Ninth Revision, Clinical Modification codes for OA between January 1, 2004 and December 31, 2007. The prevalence and the adjusted hazard ratio (HR) of dementia in patients with and without OA were estimated. The OA cohort comprised 35,149 patients and the non-OA cohort (comparison cohort) comprised 70,298 patients (1:2). The incidence of dementia was 21.7 per 10,000 person-years in the OA cohort and 14.7 per 10,000 person-years in the non-OA cohort. The HR for dementia during the follow-up period was 1.33 (95% confidence interval [CI], 1.17-1.50, P < 0.001) for patients with OA. The adjusted HR for dementia was 1.25 (95% CI, 1.10-1.43, P < 0.001) for patients with OA. The results of this study indicated that OA is an independent risk factor for dementia.