Add vodka, lemon juice, simple syrup, and watermelon juice to a cocktail shaker.
Fill the shaker three-quarters with ice and shake hard for about 15 seconds until well chilled.
Fine-strain into a chilled rocks or coupe glass.
Optional: Top with a splash of club soda for a little fizz.
Garnish with a small watermelon wedge.
Notes
Use ripe, fresh watermelon for clean flavor; double strain for a silky texture. Adjust simple syrup to taste depending on fruit sweetness. Serve immediately while cold.