Use a blender or food processor to puree the cubed watermelon until smooth and vibrant pink.
Pour the watermelon puree into a large, heavy-bottomed pot and bring to a gentle simmer over medium heat, stirring occasionally. Cook until the mixture reduces slightly and begins to thicken, about 10 minutes, releasing a fruity aroma.
Gradually stir in the sugar and lemon juice, stirring continuously until the sugar dissolves completely and the mixture becomes glossy. Continue to cook over medium heat, stirring regularly with a wooden spoon.
Add the fruit pectin to the mixture and stir vigorously to ensure even distribution. Bring the mixture to a rolling boil—a boil that can't be stirred down—for about 1 minute, or until it thickens and coats the back of a spoon.
Remove the pot from heat and test the setting point by placing a small spoonful on a chilled plate. Once cooled, it should firm up slightly. Ladle the hot jam into sterilized jars, leaving about ½ inch headspace. Seal the jars with lids and allow to cool completely at room temperature.
Label and store the jars in a cool, dark place. Refrigerate after opening. The jam will have a sticky, jelly-like texture with a bright pink color and can be used on bread, scones, or as a topping.
