Studies investigating the risk for dementia in psoriatic patients remain inconclusive. There are a limited number of population-based studies on the association between psoriasis and dementia. This study aims to investigate the risk for dementia in psoriatic patients. This nationwide population-based cohort study was based on data obtained from the Taiwan National Health Insurance Research Database between 2000 and 2012. A total of 111,825 patients with psoriasis and 111,825 age-, sex-, and index date-matched controls were recruited. The hazard ratio (HR) for subsequent dementia in patients with psoriasis was analyzed using a Cox model and a Fine-Gray competing risk model. During 1,358,774 person-years of follow-up, 2688 patients developed dementia in the psoriatic cohort, and 2062 developed dementia in the control cohort. In the multivariate adjusted Cox model, the hazard ratio (HR) of psoriatic patients for dementia was 1.02 [95% confidence interval (CI) 0.96–1.09] relative to the controls. Psoriasis did not increase the risk for dementia (both vascular dementia and degenerative dementia). In the competing risk model, the HR of dementia was 0.96 (95% CI 0.90–1.02) for psoriatic patients. Compared to psoriatic patients who had not received phototherapy or systemic treatment, those psoriatic patients receiving phototherapy or systemic treatment had a lower risk for dementia. However, this phenomenon was not observed in psoriatic patients who were observed for longer than 6 years. Psoriasis was not found to be a risk factor for dementia. Phototherapy and systemic treatment might not have a protective effect against dementia in psoriatic patients.