Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.
Place rhubarb and strawberries in baking pan. Evenly spread strawberry-flavored gelatin mix over fruit, then repeat with cake mix. Pour water over cake mix layer; do not stir. Dot the top of the cake with butter.
Bake in the preheated oven until cake is bubbly, about 20 minutes.