Cut the watermelon into chunks, removing the seeds and rind.
Place the watermelon chunks in a blender or food processor and blend until smooth.
Pour the pureed watermelon into a large pot or saucepan and bring it to a boil over medium-high heat.
Once boiling, reduce the heat to low and let the watermelon simmer for about 1-2 hours, stirring occasionally, until it has thickened to a jam-like consistency.
Keep an eye on the jam and stir more frequently as it thickens to prevent it from sticking to the bottom of the pot.
Once the watermelon jam has thickened, remove it from the heat and let it cool completely.
Transfer the cooled jam into sterilized jars or containers and store them in the refrigerator.
Enjoy your homemade sugar-free watermelon jam on toast, pancakes, yogurt, or as a topping for desserts!