Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the cooked rotini, shredded chicken, broccoli, and Alfredo sauce.
Add garlic powder, Italian seasoning, salt, and black pepper to the mixture; stir until well combined.
Transfer the mixture to a greased casserole dish, spreading it evenly.
Sprinkle the top with mozzarella and Parmesan cheeses.
Cover the casserole with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let it sit for 5 minutes before serving.