Place the cubed watermelon into a blender or food processor.
Squeeze the juice of lemons into a cup, and set aside.
Blend the watermelon cubes at high speed until smooth and well combined. Use a fine mesh strainer to get the watermelon pulp.
Add a sweetener of your choice, such as sugar, honey, or stevia, to the watermelon. I used regular sugar. Mix until the sugar dissolves. Adjust to your preference.
Pour the lemon juice into the sweetened watermelon juice.
Garnish with a slice of watermelon or mint (optional).
Serve immediately and enjoy!