Cut ripe watermelon into chunks, then blend in a blender until smooth. Strain the puree through a fine mesh strainer into a bowl to extract clear juice, discarding solids.
Pour the strained watermelon juice into a large saucepan. Add granulated sugar and lemon juice, stirring gently to combine. Place the saucepan over medium heat and bring the mixture to a gentle simmer.
4 cups watermelon juice
Stir continuously with a wooden spoon, watching for the mixture to become translucent and slightly thickened. Use a cooking thermometer to monitor temperature; cook until the mixture reaches about 220°F (105°C), indicating jam consistency.
Maintain a gentle simmer, occasionally scraping the sides of the pan to prevent caramelization. The jam should start to thicken and take on a shiny appearance, with bubbles becoming slower and larger.
Once the desired thickness is achieved, remove the saucepan from heat. Pour the hot jam carefully into sterilized jars, leaving a small headspace, and seal tightly. Allow the jam to cool at room temperature.
Label the jars once cooled, and store in a cool, dark place. The jam will set further as it cools, resulting in a glossy, chunky spread with a bright pink color and floral aroma.
