Atrial fibrillation (AF) is a common cardiac arrhythmia. There is growing evidence that AF is a risk factor for cognitive decline and dementia. In this review, we summarise epidemiological observations linking AF with cognitive outcomes, describe potential mechanisms and explore the impact of AF treatments on cognitive decline and dementia. Community-based observational studies show a consistently higher rate of cognitive decline and increased risk of dementia in persons with AF. These associations are partly due to the increased risk of clinical stroke in AF, but other mechanisms, including the incidence of silent cerebral infarcts, microbleeds and cerebral hypoperfusion, are likely additional contributors. Adequate oral anticoagulation and improved management of the overall cardiovascular risk profile in individuals with AF offer the promise of reducing the impact of AF on cognitive decline and dementia.