Use a sharp knife and cutting board to chop the watermelon into small cubes, then transfer to a blender or food processor.
Puree the watermelon until smooth, then pour the mixture through a fine sieve or cheesecloth to remove excess pulp and achieve a smooth consistency.
Pour the strained watermelon puree into a large saucepan and heat over medium heat. Bring to a gentle simmer while stirring regularly with a wooden spoon or silicone spatula, until the mixture begins to thicken and becomes slightly translucent, about 10 minutes.
Add the sugar and lemon juice to the saucepan, stirring constantly until the sugar dissolves completely. Continue simmering until the mixture thickens further and coats the back of a spoon, about 15-20 minutes. The jam should be wobbly but hold its shape.
Remove the saucepan from heat and transfer the hot jam into sterilized jars. Seal immediately and let cool to room temperature before storing in the refrigerator for up to 2 weeks or processing in a water bath for longer preservation.
