Place watermelon chunks in a blender and blend until smooth.
Add mint leaves and simple syrup to a Collins glass. Then, muddle gently to release the mint oils.
Fill the glass 2/3 full with ice cubes.
Pour in the rum, lime juice, and 2 oz of watermelon juice.
Top with club soda and stir gently.
Garnish with a watermelon wedge and fresh mint sprig.
Notes
Store extra watermelon juice in an airtight container for up to 2 days.For a mocktail version, skip the rum and add extra club soda.Use fresh watermelon for the best flavor. Frozen watermelon makes the drink too watery.Make simple syrup by heating equal parts sugar and water until dissolved, then cool completely.