Scientists said including it in diets helps keep the brain young.
The superfood is rich in cell-protecting antioxidants and known for its multiple health benefits.
That includes boosting brain function - key to slowing down effects of ageing.
The US researchers previously found the oil preserves memory and protects the brain against Alzheimer's disease. Their latest findings are the first to suggest it can also defend against mental decline known as frontotemporal dementia.
Senior investigator Professor Domenico Pratico, of Temple University in Philadelphia, said: "Extra virgin olive oil has been a part of the human diet for a very long time and has many benefits for health, for reasons we do not yet fully understand.
"The realisation that it can protect the brain against different forms of dementia gives us an opportunity to learn more about the mechanisms through which it acts to support brain health."
Alzheimer's, a form of dementia, primarily affects the brain's memory storage. Frontotemporal dementia includes personality and behaviour changes.
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