Toss the watermelon and sugar together until the sugar is completely moistened and evenly distributed.
Allow the watermelon mixture to sit at room temperature for 2 hours, tossing halfway through.
After 2 hours, strain the watermelon syrup from the solids using a fine mesh strainer. Reserve the syrup, discard or save the watermelon solids for another use.
Transfer the watermelon syrup to an airtight container or syrup bottle, and store in the fridge for up to 7 days.