Place diced rhubarb in the bottom of a greased 9x13 pan. Sprinkle Jello, sugar, and water on top of rhubarb.
In a medium sized bowl, mix together the cake mix and melted butter. Once loosely combined, crumble on top of rhubarb mixture.
Bake at 350 degrees for 1 hour or until top browns.
Serve warm with cool whip or ice cream. For a fun presentation, spoon mixture in small dessert glasses and top with whipped cream or ice cream. Makes for a tasty seasonal treat!
Gluten Free
You can replace the cake mix with a gluten free cake mix. Keep in mind that they are typically half the size of a regular cake mix, so it may take 2.