Preheat oven to 375°F. Lightly grease an 8-inch-square baking dish. Spoon 1/4 cup marinara sauce over bottom of dish. Cover with half of ravioli. Spread half of remaining sauce over ravioli. Sprinkle on spinach and then half of mozzarella. Repeat with remaining ravioli, marinara and mozzarella. Sprinkle top with Parmesan.
Cover dish with foil and bake for 30 minutes. Remove foil and bake until bubbling, about 10 minutes longer. Let cool for about 5 minutes before slicing.