Combine sugar, water, and lemon juice in a saucepan over medium heat; cook and stir until sugar is dissolved, about 5 minutes. Remove from heat and refrigerate until cooled, about 30 minutes.
Blend watermelon in a blender or food processor until pureed. Stir pureed watermelon into sugar mixture. Transfer watermelon mixture to an ice cream maker and freeze according to manufacturer's instructions.
