Preheat oven to 350°F. Spray a 9×13 baking dish with cooking spray.
Mix cherries and pineapple in a 9×13 baking dish. Sprinkle dry cake mix over pineapple and cherry mixture; stir until just combined.
Sprinkle walnuts over the top.
Cut the butter into thin slices and place it all over the top.
Bake for 35-40 minutes or until golden brown.
Studded with pops of fruit and crunchy walnuts, this cherry dump cake recipe is a perfect last-minute dessert!