They said that while we may find it hard to believe, a few cups of coffee a day is as hydrating as water.
The Birmingham University researchers called for health advice to be updated to reflect their findings.
Sports scientist Sophie Killer said that the idea that coffee dehydrates stems from research done on samples of caffeine over 80 years ago.
However, caffeine and the many other compounds in coffee interact with each other and so the 1928 study isn’t necessarily relevant to daily life.
Miss Killer said: ‘It is estimated that 1.6billion cups of coffee are consumed worldwide every day, thus it is of interest to know whether coffee contributes to the daily fluid requirement or whether it causes low-level chronic dehydration.’
To find out, the researcher asked 50 healthy men to drink either four mugs of water or coffee a day for three days and then switch.
The men ate the same food during the two parts of the study and were banned from vigorous exercise and alcohol.
Tests of blood and urine samples showed the men were just as well hydrated when they drank coffee and when they had water.
They also passed the same amount of urine, the journal PLOS ONE reports.
Forget what you may have heard -- despite caffeine's effects in making you want to urinate, a small new study shows that moderate coffee consumption does not cause people to become dehydrated.
The research was funded by the Institute for Scientific Information on Coffee, which is a nonprofit organization. It should be noted that European coffee companies -- including illycaffe and Nestle -- are members of the ISIC. However, ISIC did not have a role in the study design or analysis.
The study, published in the journal PLOS ONE, involved having 50 men drink four mugs of black coffee or four mugs of water a day for three days. Then, after a 10-day "wash-out" period, the men who initially drank