Does drinking coffee dry out your skin?
Drinking coffee does not dry out your skin: the available research points rather in the opposite direction, but the studies are still limited in size, so do not draw sweeping conclusions from them.
Coffee polyphenols appear to help the skin rather than harm it. In a double-blind study of 49 women with mildly dry skin, daily intake of coffee polyphenols (270 mg per day for eight weeks) visibly improved skin hydration: dryness scores fell, transepidermal water loss decreased, and the outermost layer of the skin retained more moisture. Skin dryness therefore actually decreased, rather than increased.
The popular notion that 'coffee dehydrates you' is based on the mildly diuretic effect of caffeine. That effect does exist: at a high dose of caffeine (6 mg per kilogram of body weight, considerably more than a normal cup of coffee) during intense exercise in the heat, urine output increased by 28%. However, in trained athletes this did not lead to measurable dehydration or a disrupted fluid balance. The effect on the skin was not measured at all in that study.
Observational research also shows that coffee drinkers take in more fluid on average than non-coffee drinkers, simply because a cup of coffee is also just water.
For completeness: there is also research into coffee ingredients applied directly to the skin. An ingredient derived from coffee silverskin and a compound called kahweol showed positive effects on skin hydration in small studies and laboratory research respectively. However, these concern skin creams and cell cultures, and say nothing about what happens in the body when you drink coffee.
Based on one RCT (n=49), one intervention study in athletes, one observational study, and two early studies (cosmetic/lab). No large-scale RCTs. The findings on polyphenols are positive but limited in scope.