Place the watermelon cubes into a large, heavy-bottomed pot.
Bring the watermelon to a gentle simmer over medium heat, stirring occasionally with a wooden spoon or silicone spatula until the fruit is soft and begins breaking down, approximately 15-20 minutes. You'll notice the liquid becoming more vibrant and syrupy.
Add the sugar and lemon juice to the mixture, stirring until the sugar dissolves completely. Continue simmering over medium-low heat, stirring frequently, until the mixture thickens and reduces by about half, which will take approximately 30-40 minutes. Look for the jam to become glossy and to pass the plate test.
Optional: For a smoother consistency, strain the mixture through a fine mesh strainer or cheesecloth, pressing gently to extract excess liquid. Return the strained jam to the pot and simmer briefly to ensure it reaches desired thickness.
Transfer the hot jam into sterilized jars, filling to within ¼ inch of the rim. Seal the jars and allow to cool to room temperature. The jam will firm up further as it cools, resulting in a shiny, gel-like spread with visible bits of crushed watermelon.
