Place the diced watermelon in a large mixing bowl and mash briefly with a fork to break down the flesh, then transfer to a blender. Blend until smooth and pour through a fine sieve into a saucepan to remove any pulp or fibrous parts, resulting in a clear, pink juice.
Add the sugar, lemon juice, and pectin to the saucepan with the watermelon juice. Stir well to combine.
4 cups watermelon flesh, diced
Place the saucepan over medium heat and bring the mixture to a gentle boil, stirring constantly with a wooden spoon or heat-resistant spatula. Continue until the mixture begins to thicken and visually coats the back of a spoon after about 10-15 minutes, with a slight glossy appearance.
Once the jam has thickened, use a thermometer to check for a temperature of around 220°F (105°C), or perform the sheet test: place a spoonful on a chilled plate, and if it wrinkles when pushed, it's ready. Remove from heat immediately.
Pour the hot jam into sterilized jars, filling them up to about ¼ inch below the rim. Wipe the rims clean, secure the lids, and allow the jars to cool at room temperature.
Once cooled, store the jars in a cool, dark place. The jam will set further as it cools, developing a smooth, glossy texture and vibrant pink color.
