Preheat the oven to 350 degrees F. Lightly grease a 9 inch by 13 inch baking pan with cooking spray.
Place the rhubarb, pineapple & sugar into a large bowl. Stir until combined. Pour mixture into the prepared pan. Sprinkle cake mix over rhubarb mixture. Dice butter & sprinkle over cake mix. Bake for 30-40 minutes or until top is golden brown. Let cool for 15 minutes. Spoon into bowls & garnish with whipped cream or ice cream.