Put pureed watermelon, lemon juice, and sugar in a pot.
Bring to a rolling boil, then reduce heat to simmer.
Add the pectin and whisk it with a handheld whisk or a fork immediately, so it dissolves without clumping.
Bring the watermelon jam to broil again, then reduce heat to simmer and simmer, stirring every minute, for 20 minutes.
Pour the watermelon jar into a mason jar. This watermelon jam will set completely once it’s cooled.
This watermelon jam can be canned, or you can simply store it in a refrigerator. (It will stay fresh in a refrigerator for up to 1 month)