Cut fresh watermelon into chunks, removing seeds and rind, then blend until smooth to extract 4 cups of juice. Pour the juice through a fine-mesh sieve or blend again briefly to ensure smoothness.
4 cups watermelon juice (freshly pureed)
Pour the watermelon juice into a large, heavy-bottomed pot over medium heat. Add the sugar, citrus zest, and lemon juice, stirring continuously until the sugar dissolves completely and the mixture starts to simmer.
4 cups watermelon juice (freshly pureed), 1.5 cups sugar, 1 tbsp citrus zest, 2 tbsp lemon juice
Increase heat slightly to bring the mixture to a gentle boil, then reduce to medium-low. Continue to cook, stirring frequently, until the mixture thickens and begins to gel, about 20-30 minutes. You'll notice the color deepening and the surface showing gentle bubbling with less liquid.
Test the jam’s consistency by placing a small spoonful on a chilled plate; if it wrinkles after a minute or two, it’s ready. Remove from heat and let cool slightly.
Pour the hot jam into sterilized jars or containers, seal tightly, and allow to cool completely at room temperature. Store in the refrigerator or process in a water bath for longer preservation.
