Cook the rotini pasta according to package instructions until al dente. Drain and set aside.
In a large mixing bowl, combine the cooked rotini pasta, marinara sauce, ricotta cheese, Italian seasoning, garlic powder, salt, and pepper. Mix well until all the pasta is evenly coated.
In a greased 9x13 inch baking dish, layer half of the pasta mixture at the bottom. Sprinkle half of the shredded mozzarella cheese and half of the pepperoni slices on top.
Add the remaining pasta mixture on top, followed by the rest of the mozzarella cheese and pepperoni.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
After 20 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 cool for a few minutes before serving.