Decaffeinated coffee is not a diuretic. It is about equal to water in how much it makes a person urinate. Caffeine is the part of coffee that acts as a diuretic.
Caffeine is not a strong diuretic; it takes more than 1 cup for it to have an effect on the body. Studies show it takes as much as 360 milligrams of caffeine to act as a diuretic. A typical 8-ounce cup of coffee has 95 to 200 milligrams of caffeine, depending on the brew. This means that it takes at least 2 cups for it to act as a diuretic. Typically, the diuretic effect is mild.