Believe it or not, watermelons are actually vegetables and members of the cucumber and squash family. Regardless of what you call them though, watermelons place second only to tomatoes in the home garden for highest concentrations of lycopene. Lycopene is an antioxidant that reduces the risks of many diseases, including some cancers and cardiovascular diseases. So eat up your watermelon and reap its benefits. But alas, this may be difficult if your watermelon is not ripe. While it's best to let watermelon ripen in the field, you have one last resort to help it along if necessary.
Leave the watermelon alone to ripen on the vine. The biggest indicator that a watermelon is ripe is the bottom where it sits on the ground. It must be a cream or yellow color, not green or white. Other signs that a watermelon is ripe include: a pale green color between the stripes, a dry and curvy tendril, a smooth rind and round edges.
Wrap a picked, unripe watermelon in a paper bag. This could help it ripen some if you inadvertently obtained a melon that does look ripe.
Store it at room temperature, not in refrigerator. Check every few days to see if the watermelon has ripened.