Preheat oven to 350 degrees. Place rhubarb in a greased 13x9 baking pan. Sprinkle with the gelatin, sugar and marshmallows.
In a large bowl, prepare cake mix according to package directions, pour batter over marshmallows.
Bake at 350° until a toothpick inserted in the center comes out clean, 40-45 minutes. Cool for 10 minutes, then invert the cake onto a serving plate.