Prepare a water bath canner by filling it with water and bringing it to a simmer over medium heat. Wash your canning jars and lids.

Cut watermelon into chunks, remove seeds, and blend until smooth. Strain through a cheesecloth-lined sieve to yield about 4 cups of juice.

Combine watermelon juice, bottled lemon juice, and sugar in a large pot. Stir over low heat until sugar is fully dissolved.

Increase heat to medium-high and bring to 190°F, stirring frequently. Do not let it boil.

Ladle the hot mixture into prepared jars, leaving about 1/4 inch headspace. Wipe rims and secure lids.

Process jars in the water bath canner for 15 minutes (20 minutes if above 1,000 feet altitude).

After processing, let jars cool for 12 to 24 hours. Check seals by pressing centers of lids.