Use a sharp knife and cutting board to peel the watermelon, removing the outer green rind and pith, then chop the pink flesh into small, uniform pieces approximately 1-inch in size.
Place the chopped watermelon rinds and flesh into a large saucepan. Pour in enough water to cover the pieces and bring to a gentle simmer over medium heat. Cook for about 15 minutes until the rinds are tender when tested with a fork.
Drain the cooked watermelon mixture through a fine strainer or colander, reserving the liquid. Return the drained watermelon back to the saucepan.
Add sugar and lemon juice to the watermelon in the saucepan. Place over medium heat, stirring continuously with a wooden spoon until the sugar dissolves completely and the mixture begins to thicken, about 20-30 minutes. You will notice bubbling and a glossy surface as it reduces.
Increase heat slightly and cook for an additional 10-15 minutes until the jam reaches a gel-like consistency, testing by placing a small dollop on a cold plate—if it holds its shape without running, it's ready.
Pour the hot jam into sterilized jars using a jar funnel. Seal the jars and allow them to cool at room temperature, then store in the refrigerator or a cool dark place for up to several months.
