Watermelons contain about 93 percent water, according to The Ohio State University Extension website, but that high water content doesn't mean that they stay fresh indefinitely at room temperature; the carbohydrates and sugars in the fruit lead to spoilage eventually. How long a melon can stay at room temperature before you refrigerate it and before it goes bad depends on its size and its degree of ripeness when you bring it home from the store.
If you plan to use a watermelon within a few days, choose the ripest one you can find; look for a yellowish-white resting spot, as opposed to a white or pale green spot, where the melon has rested on the ground. A soft "give" when you press the blossom end of the melon, opposite the stem end, also indicates ripeness.
Whole melons stay fresh at room temperature, preferably in a cool, dark place to retard further ripening, for up to 10 days under ideal conditions, with no additional wrapping needed. But if your watermelon is already fully ripe when you bring it home, it may start to spoil within 3 or 4 days, especially on hot summer days.
A whole watermelon stays fresh for up to 2 weeks in the refrigerator, with no wrapping necessary, but may only last for 1 week depending on the degree of ripeness when purchased or picked.
Tips
Let watermelon come to room temperature for 1 hour before serving so its flavors and aromas are more intense.
Once it's cut, whether into chunks or slices without the rind or in halves or quarters with the rind left on, watermelon stays fresh in the refrigerator for 3 to 4 days; never store cut watermelon, or any cut fruit, at room temperature for longer than 2 hours. Cover the cut portion of the melon tightly with plastic or foil or store cut pieces in an airtight container to slow down the aging, oxidation processes.
Tips
Cut watermelon stays safe for up to 10 months wrapped tightly in foil and plastic in the freezer. The flesh will be soft and inedible for eating out of hand, but it adds sweet, watermelon flavor in smoothies and sorbets.