No at normal intake: a meta-analysis found caffeine raises urine output only slightly, the effect shrinks further during exercise, and regular drinkers develop tolerance.
Coffee is mostly water - the fluid it delivers far outweighs the small extra urine, so daily cups count toward hydration, not against it. Boundaries: very high doses do measurably increase excretion, and first-time drinkers get a stronger diuretic effect than habituated ones. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper .
What to do
Common questions
Does coffee dehydrate you?
No at normal intake - the diuretic effect is real on paper and trivial in the cup. A meta-analysis of caffeine and urine output found that while acute caffeine ingestion can increase urine volume slightly, the effect is small and effectively disappears during exercise, and tolerance builds in regular drinkers. The full-day math matters more: coffee is mostly water, and the fluid it delivers far outweighs the small extra urine it provokes, so your daily cups count toward hydration, not against it. The honest boundaries: very high caffeine doses do increase fluid and electrolyte excretion measurably, and if you never drink coffee, the first strong cup acts more diuretic than it would in a habituated drinker. Practically: coffee contributes to your fluid intake, the dehydration warning is a myth at typical doses, and the symptoms people blame on "coffee dehydration" - dry mouth, headache - are more often just under-drinking water alongside it. Keep a glass of water near the cup and the question answers itself.