Add the diced dragon fruit and 1 cup of cold water to a blender. Blend until smooth.
Strain the dragon fruit mixture through a fine mesh strainer for a smoother drink, or leave it unstrained for a fruitier texture.
Pour the dragon fruit mixture into a large pitcher.
Stir in the fresh lemon juice, remaining 3 cups cold water, sweetener, and a tiny pinch of salt.
Taste and adjust. Add more lemon juice for tartness, more sweetener for sweetness, or more water if the flavor is too strong.
Chill for at least 30 minutes.
Serve Dragon Fruit Lemonade over ice with lemon slices, diced dragon fruit, and fresh mint.