Add the watermelon cubes to a blender and blend until smooth.

Strain the watermelon puree through a fine-mesh sieve to remove any pulp.

In a small saucepan, combine the sugar and ½ cup water. Bring to a gentle boil and stir until the sugar dissolves completely.

Remove from heat and mix in the 3 cups cold water and fresh lemon juice.

Pour the watermelon juice into the lemonade mixture and stir well.

Fill a large pitcher with ice cubes and pour the watermelon lemonade over the ice.

Garnish with lemon slices or fresh mint if desired, and serve chilled.