The Short Version
This claim is false. The scientific consensus, supported by peer-reviewed meta-analyses and major health authorities like the NHS, is clear: moderate coffee consumption does not cause dehydration in healthy adults. Coffee's mild diuretic effect is transient and far outweighed by the water content of the beverage itself. The only studies showing negative fluid balance used extreme caffeine doses in caffeine-deprived subjects—conditions irrelevant to normal coffee drinking. Regular consumers develop tolerance to caffeine's diuretic effects.