Place your martini glass in the freezer for at least 10 minutes before making the cocktail. This helps keep the drink extra cold and refreshing.
In a cocktail shaker, add the fresh watermelon and mint leaves. Use a muddler to gently mash everything together until the watermelon releases its juices and the mint becomes fragrant.
Pour in the vodka, triple sec, lime juice, and simple syrup.
Add ice to the shaker and shake vigorously until the cocktail is well chilled.
Remove the martini glass from the freezer and strain the cocktail into the chilled glass.
Garnish with fresh mint leaves and enjoy immediately.