Fill a large canning pot with water and set it over high heat to bring to a boil. Place clean jam jars inside the pot to warm them up.

Cut half a large watermelon into smaller pieces and blend until smooth, then strain to separate juice from pulp.

In a large pot, combine watermelon juice, chopped strawberries, lemon juice, and butter. Add pectin if using.

Place the pot over medium-high heat and stir frequently until it reaches a rapid boil.

Add sugar gradually once boiling, stirring constantly for 1 minute and 10 seconds.

Remove jars from the pot and fill with hot jam, leaving ¼ inch space at the top. Wipe rims and seal with lids.

Process jars in boiling water for 10 minutes to create a vacuum seal. Allow jars to cool at room temperature for 24 hours.