Preheat the oven to 350 degrees F (175 degrees C). Grease a 9x13 inch pan.
In a large bowl, stir together the rhubarb, sugar, marshmallows and dry strawberry flavored gelatin. Pour into the bottom of the prepared pan, and distribute evenly. Mix cake batter according to package directions; pour over the ingredients in the pan.
Bake for 1 hour in the preheated oven, until a toothpick inserted into the center comes out clean. Run a knife around the outer edge of the cake to loosen; turn out onto a large serving plate while still hot. Allow the cake to cool before serving.