Cut the watermelon into 1-inch cubes, removing seeds as you go. Pat dry with paper towels if very juicy and place in a large serving bowl.
Crumble the feta cheese into bite-sized pieces and add to the bowl with watermelon.
In a small bowl, whisk together olive oil, lime juice, salt, and pepper until well combined.
Pour the dressing over the watermelon and feta, then gently toss to combine without breaking up the watermelon pieces.
Add the chopped mint leaves and give one final gentle stir.
Serve immediately, or refrigerate for up to 24 hours to allow flavors to meld. Stir gently before serving if made ahead.
Notes
Storage tip: This salad keeps in the refrigerator for up to 3 days and actually improves with time. For best texture, add fresh mint just before serving if making more than 4 hours ahead. Can be made dairy-free by substituting feta with toasted seeds and extra salt.