Cut the watermelon into 1/4-inch pieces, carefully removing all seeds to ensure smooth texture in the finished jam. Measure and arrange all remaining ingredients near your cooking area for easy access once heating begins.

Add the diced watermelon, sugar, and lemon juice to a large heavy-bottomed pot (at least 4-5 quart capacity). Stir to combine, then heat over medium-high, stirring occasionally to prevent sticking, until the mixture reaches a rolling boil. This step releases the watermelon's juices and dissolves the sugar evenly.

When the mixture is at a rolling boil, lower the heat to medium and gradually sprinkle the pectin over the surface while stirring constantly for 1-2 minutes until fully dissolved with no lumps. Once the pectin is incorporated, bring back to a full rolling boil and add the salt and lemon zest.

Keep the mixture at a full, rolling boil for exactly 1 minute, stirring constantly to activate the pectin. Then lower the heat to medium-low and simmer gently for 20 minutes, stirring frequently to prevent scorching. The jam should darken and thicken during this time. Test for doneness by placing a spoonful on a cold plate—it should wrinkle slightly when pushed with your finger.

Take the pot off the heat and allow the jam to rest for 2-3 minutes so foam can settle. Pour the hot jam into clean, sterilized jars, leaving about 1/4 inch of headspace. Wipe the jar rims clean, seal with lids, and let cool completely at room temperature for at least 6-8 hours. The jam will continue setting as it cools and reach its final texture by the following day.