That's the finding of a study reported in the Heart Rhythm Journal.
Researchers followed more than 37,000 people without atrial fibrillation or dementia over a five-year period. They found the risk was greatest for those under the age of 70.
The authors believe both atrial fibrillation and dementia may arise from the same risk factors, such as hypertension.
Another possibility is that atrial fibrillation increases inflammation.
Dementia has been found to be higher in people with signs of systemic inflammation.