Add the sugar and rhubarb to a pan with 3 cups of cold water. Bring to a boil over high heat, then reduce to a simmer. Simmer, uncovered, until the rhubarb softens and the syrup thickens slightly, 15 to 20 minutes.
Strain the syrup through a fine-mesh strainer, making sure to press all of the liquid out of the rhubarb (see Cook's Note). Allow the syrup to cool completely, then pour into an airtight container. Store in the refrigerator for up to 2 weeks.
The smashed rhubarb can be reserved for later use as a jam or spread!