Preheat oven to 350 degrees F (175 degrees C). Prepare a 11x7x2-inch baking dish with non-stick cooking spray.

Spread rhubarb into the baking dish. Sprinkle white sugar, gelatin mix, and yellow cake mix over the rhubarb. Pour water over the rhubarb mixture. Drizzle butter over the rhubarb mixture.

Bake in preheated oven until rhubarb is tender, about 50 minutes. Cool on a wire rack for 15 minutes. Dust with confectioners' sugar and top with whipped topping.