Researchers found that older people with hypertension suffered a drop in IQ and the ability to recall simple information on days when there blood pressure was particularly high.
While the exact link between high blood pressure and reduced mental ability is unknown, scientists said the research showed that sufferers should be extra vigilant about managing their condition and avoiding stress.
"This means that stressful situations may make it more difficult for some seniors to think clearly," said Dr Jason Allaire, an assistant professor of psychology at North Carolina State who co-authored the study.
He suggested the effect can be witnessed in people in their 50s, when many people develop high blood pressure and cognitive ability begins to decline.
"If you have high blood pressure it is really important to get it under control."
He said that although a little stress is considered good, in older people already suffering from high blood pressure it could be "bad news" as it can cause a sharp rise in pressure levels.
The research could help suggest why some elderly people become confused when under stress, he added.
The team tested 36 elderly people, aged between 60 and 87, all with high blood pressure, twice a days for 60 consecutive days. After having their blood pressure taken they were asked to carry out a number of tests including simple memory recalls, number recognition and an IQ test.
The results of the test showed a direct correlation between the times when their blood pressure was high and lower "cognitive" ability. The higher the blood pressure the more mistakes were made.
Dr Allaire said that the effect only occurred with people with higher than average blood pressure to start with.
The study subjects whose average blood pressure was 130 or higher ( the healthy upper limit is 120) saw a significant decrease in cognitive function when their blood pressure spiked, said the paper published in Journal of Gerontology: Psychological Sciences.
However study subjects whose average blood pressure was low or normal saw no change in their cognitive functioning – even when their blood pressure shot up.