In a large bowl, combine the cubed watermelon, crumbled feta cheese, sliced red onion, and chopped mint leaves.
In a small bowl, whisk together the olive oil, balsamic vinegar, salt, and pepper until well combined.
Drizzle the dressing over the watermelon mixture and gently toss to combine, being careful not to break up the feta too much.
Chill the salad in the refrigerator for at least 15 minutes before serving to allow the flavors to meld.
Serve the salad cold, garnished with additional mint leaves if desired.