Aging is the primary risk factor for Parkinson’s disease (PD) and cognitive impairment from PD is a major and unmet biomedical challenge. Klotho, a pleiotropic protein, extends lifespan and enhances cognition. Whether longevity factors such as klotho can counteract PD-related mortality and deficits in mice or associate with resistance to PD in humans is unknown. Here we show that transgenic elevation of klotho increased lifespan, improved synaptic and cognitive, but not motor, functions in mice, and decreased steady state α-synuclein levels in the brains of mice that express wildtype human α-synuclein. In humans, a genetic variant of KLOTHO that increases circulating klotho levels associated with better executive cognition and less CSF abnormalities of α-synuclein in individuals with PD. Thus, klotho can counteract cognitive deficits related to PD, possibly modulating α-synuclein levels – and these findings may be relevant to new therapeutic pathways for human PD.