Place everything into a blender and blend until smooth.
If desired, pour the juice into the fine-mesh strainer in a large bowl and press the pulp with a spoon to extract all the juice.
Serve immediately with some ice cubes or chill it for an hour in the refrigerator before serving.
Notes
Tips & Notes
Look for a super-ripe seedless watermelon for the best flavor. If your melon is not sweet then you can also add sweeteners like sugar, stevia, honey, or maple syrup while blending.
Pour leftover juice into mason jars and store in the refrigerator for up to three days. It will settle and separate a bit, just make sure to stir before serving.
To make a slushy version, you could freeze the watermelon cubes in advance and then blend them with some ice cubes.
Note: Nutrition values are my best estimates. If you rely on them for your diet, use your preferred nutrition calculator.