Preheat oven to 350 degrees F (175 degrees C).
Line the bottom of one ungreased 9×13 inch baking pan with the diced rhubarb. Sprinkle the white sugar over the top. Then sprinkle the raspberry gelatin over the top of the sugar. Next sprinkle the cake mix over the top of the gelatin.
Melt the butter or margarine and pour it over the top of the cake mix. Pour the water over the top of the cake.
Bake at 350 degrees F (175 degrees C) for 45 minutes.