Place the horchata in the freezer for about an hour before making the smoothie so it's very cold. Keep the melon in the refrigerator so it's chilled as well.

Cut the melon into pieces. Add the melon and horchata to a blender. Blend well until the mixture is smooth and frothy.

Taste for sweetness and add sugar if needed. Pour into a glass, add a straw, and serve cold.