Use a chef's knife to dice the watermelon into small, uniform pieces, removing any seeds. Measure out 3 cups of diced watermelon and set aside.
Pour the diced watermelon into a medium saucepan. Add the granulated sugar and lemon juice, then stir to combine evenly.
Place the saucepan over medium heat. Bring the mixture to a gentle simmer, stirring frequently with a wooden spoon to dissolve the sugar and prevent sticking.
Cook the mixture, stirring occasionally, until it thickens and darkens slightly, about 20-25 minutes. The jam should be translucent and hold a slight jiggle when stirred.
Once the mixture reaches the desired consistency, remove the saucepan from heat. Use a ladle to transfer the hot jam into sterilized jars, leaving space at the top. Seal the jars with lids.
Allow the jam to cool to room temperature before storing. The finished watermelon jam will be glossy, slightly chunky, and can be spread or spooned onto various dishes.