However, since the same doesn't apply for coffee drinkers, the researchers say that the antioxidant compounds in black tea may be behind the effect.
During the study, the researchers reviewed the lifestyle of 2,500 people aged 55 or over, and recorded how much tea they drank.
Each volunteer underwent a test to measure their cognitive function, or the "fitness" of their brain.
Two years later, the researchers found that those who drank tea the most were least likely to suffer cognitive decline, an early sign of dementia.
Drinking two to three cups a day cuts the risk of illness by around 55 per cent, while in heavy tea drinkers - those on six to 10 cups a day - it was 63 per cent.
Coffee drinkers, on the other hand, saw either no benefit or a tiny increase in their risk of dementia, according to the study report appearing in the American Journal of Clinical Nutrition.
The researchers say that it had yet to be determined whether adding milk would reduce such effects.
It is believed that tea works its magic either through preventing oxidation, or by blocking the build-up of the brain deposits called plaques.
"Because tea is cheap, non-toxic and widely consumed, it has huge potential in promoting cognitive health and perhaps delaying dementia," the Daily Express quoted the researchers, as saying.