Combine the watermelon puree, lemon juice, and granulated sugar in a pot. Bring the mixture to a vigorous boil, then lower the heat to simmer.

Quickly add the pectin and whisk thoroughly with a handheld whisk or fork to ensure it dissolves completely without forming clumps.

Increase the heat to bring the mixture back to a boil, then lower it to simmer. Stir every minute for about 20 minutes.

Transfer the watermelon jam into a mason jar. It will fully set once it cools.

Store the watermelon jam in the refrigerator for up to 1 month or can it for longer preservation.