Preheat the oven to 350 degrees F (175 degrees C). Coat a 13x9-inch baking dish or sheet pan with cooking spray.
Layer chicken, broccoli, and potatoes evenly into the prepared baking dish in the order listed. Drizzle melted butter over the top. Sprinkle with Italian dressing mix.
Bake in the preheated oven until chicken is cooked through and potatoes are tender, 45 to 60 minutes.
