Place 2 green tea bags in a large glass or heat-safe pitcher. Pour in 1 cup of hot water. Add 10 basil leaves, stir gently, and let steep for 15 minutes.
Add diced watermelon to a blender and blend until smooth.
Strain through a fine mesh sieve to remove pulp, collecting the juice.
After the tea has steeped, remove the tea bags and basil leaves. Add ice cubes and stir for about 20 seconds to chill.
Pour in the strained watermelon juice and stir again to combine.
Garnish with extra basil leaves or a small watermelon wedge if desired.
Notes
Sweeten to taste with honey or simple syrup if desired.
For extra flavor, muddle the basil lightly before steeping.
For a sparkling version, top with club soda before serving.