Combine 4 cups chopped rhubarb, 1 cup granulated sugar, and 1 cup water in a medium saucepan with a heavy bottom and bring to a boil.

Once the mixture is boiling, reduce the stovetop temperature to its lowest heat and allow the mixture to simmer and cook gently until the rhubarb is soft and the liquid has thickened slightly, about 20 minutes. You should have a bright magenta mixture at this point.

Ladle the mixture into a fine strainer placed over a medium, heatproof boil. Strain until most of the liquid is in the bowl and give a little press on the strained, stewed rhubarb with the back of a spoon to extract a little bit more syrup.

Pour the syrup into a clean jar, cover and refrigerate. It should keep for quite some time in the fridge.