Although it is commonly believed that tea acts as a diuretic, causing more frequent urination, this is a medical myth that is not backed up by scientific evidence. Similarly, research indicates that coffee does not have a significant diuretic effect.
Studies into the effects of tea have generally focused on caffeine, which can have an effect on the kidneys if consumed in large doses. While the caffeine in tea could be considered a mild diuretic, tea consumption would need to be significant for it to have any ill-effects on hydration levels. It seems likely that people are simply aware that they need to urinate after drinking tea, but this would also be the case if they had consumed an equal amount of water.