While you can store a whole watermelon for up to 10 days at room temperature, once you’ve cut into it, you should refrigerate it for no more than four days. Any longer and you risk consuming a tasteless piece of fruit -- or worse yet, making yourself sick.
Keep It Chill
A cut watermelon should be covered with plastic wrap or put in an airtight container. Although whole watermelon ideally should be stored at 55 degrees Fahrenheit, the U.S. Food and Drug Administration says to store cut melon in a clean refrigerator with a temperature of 40 F or below to prevent bacterial contamination that can cause food poisoning. For added freshness, leave the seeds in the watermelon.