Pour the watermelon juice into a shallow glass baking dish. Add the sugar and lemon juice and stir well. Place the dish in the freezer and chill.
Remove the pan every 1/2 hour and stir. The juice will form light, grainy ice crystals as it freezes. Repeat this process until the juice is fully frozen, about 4 hours, depending on the temperature of your freezer. You can also use an ice cream maker, following the manufacturer's directions.
Serve immediately, or store in the freezer in a covered container up to several weeks.