Preheat the oven to 350°F and lightly grease a 9×13-inch casserole dish with nonstick cooking spray. Set it aside; we want it ready for our masterpiece!
Cook the spaghetti noodles according to the package directions, using well-salted water. This ensures the noodles are flavorful when we combine everything. Drain and set aside.
In a large pot, combine the undrained Rotel tomatoes, cream of chicken soup, evaporated milk, and cream cheese. Heat them over low heat, stirring constantly until the cheese has melted into a creamy sauce.
Now, add the powdered ranch dressing mix, the diced chicken, and the cooked pasta to the pot. Mix it all together until every noodle is coated with that luscious sauce.
Pour this delightful mixture into the prepared casserole dish and top it off with shredded cheddar cheese. Oh, how that cheese will melt!
Bake uncovered for 20 to 30 minutes, until the dish is heated through and bubbly as a daydream.
For an extra touch of golden brown deliciousness, place the casserole under the broiler for 3-4 minutes. Keep an eye on it; we want it perfect!
Serve hot and enjoy this comforting and delicious Chicken Spaghetti!