WebMD explains that the misconception that tea has a dehydrating effect has been proven to be a myth. Instead, tea qualifies as a hydrating beverage in the same vein as water whether it is caffeinated or not.
According to Wikipedia, the decaffeination process used to create decaffeinated tea involves the use of various solvents to remove the caffeine from the tea leaves. In all decaffeination processes used, it is estimated that there is still 1 to 2 percent of the original caffeine content left in the decaffeinated tea, leading to the myth that decaffeinated tea causes dehydration.
The BBC explains that neither decaffeinated nor caffeinated teas have any measurable dehydration effect at all. This misconception is based on a small study from 1928 that examined only three individuals drinking caffeinated tea and other beverages. The study only used three participants, and the results would have been deemed inconclusive by modern research standards.
The BBC states there are more recent studies that prove that the diuretic effect of caffeinated tea and coffee is much less significant than once believed. These studies indicate that tea of any kind has no negative effect on hydration. In fact, whether caffeinated or decaffeinated, tea is likely just as useful for hydrating the body as a glass of water or any other beverage.