Step 1

Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13-inch baking pan.

Step 2

Combine cake mix, water, eggs, and vegetable oil in a large bowl; mix until moistened. Beat with an electric mixer on medium speed until batter is smooth, about 2 minutes.

Step 3

Spread blueberries in the bottom of the baking pan. Layer sugar, gelatin, and marshmallows on top. Pour cake mix evenly over marshmallows and smooth the top.

Step 4

Bake in the preheated oven until a toothpick inserted in the center of the cake comes out clean, 50 to 55 minutes. Transfer baking pan to a wire rack and let cool, about 30 minutes. Serve cake with whipped topping.