Use a sharp knife to cut the watermelon into small cubes. Place the cubes into a measuring cup or bowl and set aside.
4 cups fresh watermelon, cubed
Add the watermelon cubes and sugar into a medium saucepan. Turn the heat to medium and stir continuously with a wooden spoon until the sugar dissolves and the mixture begins to release juice, about 3-5 minutes.
4 cups fresh watermelon, cubed
Bring the mixture to a gentle simmer and cook, stirring often, until the watermelon breaks down and the mixture thickens, about 20-25 minutes. The mixture should become glossy and start to hold together when stirred.
4 cups fresh watermelon, cubed
Add the finely grated lime zest to the saucepan and stir well. Continue to cook for another 10-15 minutes until the jam is thick, translucent, and coats the back of a spoon. You should see bubbles forming regularly, and the mixture should no longer be watery.
4 cups fresh watermelon, cubed
Remove the saucepan from heat and let the jam cool slightly. Pour it into sterilized jars or containers, then allow it to cool completely, during which it will set further and develop a shiny appearance.
Seal the jars and store in the refrigerator. The jam will keep for up to two weeks, with a glossy, thick, and spreadable texture that resembles honey.