Preheat the oven to 350°F (175°C).
Bring a large pot of lightly salted water to a boil. Cook spaghetti in the boiling water, stirring occasionally until tender yet firm to the bite, about 12 minutes. Drain.
Combine cream of mushroom soup, diced tomatoes with green chile peppers, chicken, and butter in a large bowl; add cooked spaghetti and mix until well combined.
Transfer to a baking dish and top with shredded Cheddar cheese.
Bake in the preheated oven until the cheese is bubbly, about 30 minutes.