Wash the grapefruit and cut into small pieces. Discard the seeds.
Put the grapefruit pieces and lemon juice in a medium-sized saucepan. Bring the mixture to a boil over medium heat, stirring occasionally.
Once the mixture is boiling, reduce the heat to low and simmer for 20 minutes, stirring often.
Add the sugar and stir until it is completely dissolved. Increase the heat to medium and bring the mixture to a boil.
Once the mixture is boiling, reduce the heat to low and simmer for 10 minutes, stirring often. The mixture should be thick and syrupy.
Remove the pan from the heat and stir in the butter (if using). Allow the marmalade to cool before transferring it to a jar or other airtight container.
Interesting Facts
Grapefruit is a hybrid of the pomelo and the sweet orange.
Grapefruit is high in vitamin C and other antioxidants.
Marmalade can be made with any type of citrus fruit.