Students who took a drink into the exam hall did up to 10 percent better than those who did not - the difference between a grade, the Daily Mail reported Wednesday.
Although it is unclear why a drink should help, one theory is that information flows more freely between brain cells when they are well hydrated.
Researchers said that drinking water may also calm nerves, while those who became thirsty during test time could be more easily distracted.
The study, which looked at hundreds of university students, compared whether they took a drink - such as water, coffee or cola - into the exam with their final marks.
The students' overall academic ability was then factored in, to ensure that the results were not skewed by the possibility that smarter students are also thirstier.
The study was presented at the annual conference of the British Psychological Society.
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