Add the cubed watermelon to a large mixing bowl. Sprinkle the granulated sugar evenly over the top. Toss until all of the watermelon is fully coated in sugar.
Let the mixture sit for 1 hour to allow the juices to release.
Transfer the mixture to a blender and blend until completely smooth and fully liquefied. Place a fine mesh sieve over a bowl and strain the liquid to remove pulp, pressing gently to extract as much juice as possible. Pour the strained syrup into a jar or syrup bottle. Store in the refrigerator for up to 7 days. Shake well before each use since separation is natural.