Preheat oven to 350 degrees. Grease a 9x13 inch baking dish.
Spread the rhubarb evenly in the bottom of the baking dish. Sprinkle the sugar over the rhubarb, followed by the jell-o, and finally the cake mix. Pour the water and melted butter over the top. Do not stir.
Bake for 45 minutes or until the rhubarb is tender. Serve warm or cold topped with whipped cream and a sprinkle of cinnamon.