The evidence-led answer
NST verdict
No, not in any way that matters for staying hydrated. Caffeine has a mild diuretic effect, but only at fairly large doses, about two to three cups of coffee in one go, and mainly in people who have not had caffeine for days or weeks. Even then the extra urine is small, around 100 mL on average in the research, which is far less than the fluid you just drank, so a cup of coffee or tea still leaves you with more fluid than you started with. Regular coffee and tea drinkers build up a strong tolerance and show no measurable diuretic effect from normal servings. Exercise cancels the effect entirely. Caffeinated drinks count towards your daily fluid, and you do not need to "drink a glass of water for every coffee".
Is caffeine a diuretic?
Mildly, and mostly under specific conditions. A large single dose, about 250 to 300 mg or more, roughly two to three cups of coffee, briefly increases urine output in people who have been caffeine free for days or weeks. In a pooled analysis of 16 studies, the effect at rest was real but modest. Ordinary servings, in people who drink caffeine regularly, showed essentially no diuretic effect at all.
Does that actually dehydrate you?
No. The pooled analysis put the average extra urine at about 109 mL, around 16% more than without caffeine. A mug of coffee is roughly 250 mL of fluid, so even in the scenario where caffeine does increase urine output, you still come out well ahead. And when researchers gave 50 men four cups of coffee a day for three days and compared it with water, there was no difference in total body water, 24 hour urine volume, or any blood or urine marker of hydration. Coffee hydrated them just as well as water.
What if you drink coffee every day?
Then the diuretic effect essentially disappears. A strong tolerance develops within a few days of regular caffeine intake, and habitual tea and coffee drinkers show no meaningful diuretic response to normal servings. The three day study above used men who habitually drank three to six cups a day, which is part of why coffee and water came out identical.
What about caffeine before a workout?
Exercise abolishes the effect. In the pooled analysis, caffeine taken before or during exercise produced no significant increase in urine output, compared with a small but significant effect at rest. So a pre workout coffee or caffeine supplement is not a hydration problem. One nuance: women showed a larger resting diuretic response than men in that analysis, though it was still modest and still did not amount to net fluid loss.
What this means in practice
Count your coffee, tea and other caffeinated drinks towards your daily fluid, because that is what the evidence says they are. You do not need to match every coffee with a glass of water. If you are new to caffeine, or you take a very large dose, you might notice you pee a bit more for an hour or two; that is a mild, temporary effect, not dehydration. In hot weather or during hard exercise, what keeps you hydrated is replacing sweat losses and drinking to thirst. Whether the fluid had caffeine in it is not the issue.
Practical meaning
Count coffee, tea and other caffeinated drinks towards your daily fluid, since that is what the evidence says they are. You do not need a glass of water per coffee. If you are new to caffeine or take a very large dose you may pee a little more for an hour or two, which is not dehydration. In heat or hard exercise, hydration is about replacing sweat losses and drinking to thirst, not about whether the fluid had caffeine in it.
Limitations and uncertainty
The three day trial used 50 habitual male coffee drinkers at a moderate dose, so it shows coffee hydrates like water for regular drinkers, not for first time users, women specifically, or very high doses. The review's search ended in 2002; the 2014 pooled analysis agrees. Women were more susceptible to the resting diuretic effect than men, though it was still modest with no sign of net dehydration. This covers healthy adults and ordinary intakes only.