Use a chef's knife and cutting board to remove the watermelon rind, then chop the watermelon into small chunks. Transfer the chunks to a blender or immersion blender and process until smooth, creating a bright pink puree.
Pour the watermelon puree into a clean saucepan and bring to a gentle simmer over medium heat, stirring frequently with a wooden spoon. Cook until the liquid reduces slightly and the mixture becomes fragrant, about 10 minutes.
Gradually stir in the granulated sugar and continue cooking over medium heat, stirring until the sugar dissolves completely. Once the mixture starts to simmer again, sprinkle in the pectin while stirring constantly to prevent clumping. Cook until the jam thickens and reaches a boil, about 5-8 minutes, looking for a jam-like consistency that coats the back of a spoon.
Add lemon juice to the thickened mixture and stir thoroughly. Remove the saucepan from heat and test the consistency by placing a spoonful on a cold plate—if it holds its shape without running, it is ready for jarred storage.
Pour the hot jam into sterilized jars, leaving about 1/4 inch headspace. Seal with lids and allow to cool to room temperature. Store in the refrigerator for up to three weeks or process in a boiling water bath for longer shelf life.
