Preheat oven to 350 F. Lightly grease a 9x13-inch glass baking dish with non-stick cooking spray.
Spread the thawed (if they were frozen) rhubarb pieces into the base of the prepared baking dish. Sprinkle the sugar evenly over the rhubarb.
In a small mixing bowl, cut the butter into the dry cake mix until crumbles form.
Sprinkle the crumbled cake mix combo over the rhubarb in the baking dish. Spread evenly and flatten as best you can.
Bake in the preheated oven for 45 to 50 minutes, or until the top is golden brown. Let cool slightly.
Serve warm, at room temp, or chilled - best with vanilla ice cream or whipped cream topping.