Measure out 4 cups of ripe watermelon chunks and place them into the blender.
Add 8 torn basil leaves, 1 tablespoon of vinegar, a pinch of salt, and black pepper to the blender.
Secure the lid and blend on high speed until the mixture is completely smooth and frothy, approximately 30 seconds. The color should be bright pink with visible basil flecks.
Pour the blended soup into serving bowls and place them in the refrigerator to chill for at least 30 minutes. The soup should be icy, with a soft, slushy appearance.
Once chilled, serve the soup immediately, garnished with additional basil if desired, and enjoy a refreshing, icy watermelon-flavored dish.