Add the chopped watermelon, sugar, and lemonade to a blender.
Blend until completely smooth and combined.
Strain the mixture through a fine-mesh strainer to remove any seeds or pulp, if desired.
Pour over ice and garnish with fresh lemon slices, if desired.
Serve immediately and enjoy.
Notes
Makes about 7 cups (roughly 4 servings at 1¾ cups each). Doubling for a crowd? Use 8 cups watermelon, 4 Tbsp sugar and 8 cups lemonade, and split between two pitchers."