In a saucepan, combine the chopped rhubarb, sugar, and water. Bring to a boil over medium heat, stirring occasionally until the rhubarb is soft, about 10 minutes.

Remove from heat and let cool slightly. Stir in the lemon juice.

Transfer the rhubarb mixture to a blender and blend until smooth.

Pour the mixture into an ice cream maker and churn according to the manufacturer's instructions until it reaches a soft-serve consistency.

Transfer the sorbet to a freezer-safe container and freeze for at least 2 hours to firm up.