In a 6 to 8 quart kettle or Dutch oven combine the watermelon puree, sugar and lemon juice. Bring the mixture to a full rolling boil (a boil that cannot be stirred down) over high heat, stirring constantly with a long handled wooden spoon.

Stir in pectin all at once. Return mixture to a full rolling boil; boil hard for 1 minute, stirring constantly. Remove kettle or Dutch oven from heat; skim off foam. Ladle jelly into clean hot half-pint jars, leaving 1/4 inch headspace. Wipe jar rims; adjust lids. Allow to cool completely away from drafts, then store in cool, dark place. Makes 4 half-pints.