Rabbits can eat watermelon, but only as a treat. The digestive system of a rabbit is not designed for handling a lot of sugar. The natural diet of a rabbit is high in fiber, and over consumption of fruit can lead to the gastrointestinal tract shutting down.
Rabbits rely on very high-fiber foods, such as hay and grass to keep their digestive system moving. Feeding a rabbit watermelon to the exclusion of fibrous vegetable matter can cause the rabbit's digestion to slow down or even stop entirely. Unfortunately, rabbits have a sweet tooth, so unrestricted access to watermelon can lead to a rabbit ignoring healthier foods. The large amounts of sugar in fruit can also cause harmful bacteria to overwhelm their GI tract. A safe amount of watermelon to feed a rabbit is a teaspoon per 2 pounds body weight.