Coffee is not dehydrating, but it can have a mild diuretic effect. This means that it can cause you to urinate more frequently, but the amount of water lost through urine is not enough to cause dehydration. In fact, studies have shown that coffee can be just as hydrating as water, and may even be more hydrating for some people. However, it is important to drink plenty of water throughout the day, even if you are drinking coffee. This will help to ensure that you stay hydrated.