Use a blender or food processor to puree the chopped watermelon until smooth and liquid. Pass the pureed watermelon through a fine sieve if a smoother texture is desired.
Pour the watermelon puree into a large heavy-bottomed saucepan and place over medium heat. Bring to a gentle simmer, stirring occasionally, and cook for 10 minutes to concentrate the flavors and reduce excess moisture.
Add the sugar, pectin, and lemon juice to the simmering puree. Stir continuously until the sugar dissolves completely and the mixture begins to thicken, approximately 5-7 minutes. The jam should start to pull away from the sides of the pan slightly when ready.
Increase the heat slightly and bring the mixture to a rapid boil. Boil vigorously for 1-2 minutes, or until the jam reaches the gel point—check by placing a small spoonful on a chilled plate, where it should firm up upon cooling. The jam should have a glossy appearance and surface bubbles.
Remove the saucepan from heat and pour the hot jam into sterilized jars, leaving about 0.5 inch headspace. Seal the jars with lids and let them cool at room temperature until set, about 1-2 hours.
Store the jars in a cool, dark place. Once cooled, the jam will have a thick, jelly-like consistency with a vibrant pink color and a smooth, glossy surface.
