Shinjuku , officially called Shinjuku City, is a special ward of Tokyo, Japan. It is a major commercial and administrative center, housing the northern half of the busiest railway station in the world as well as the Tokyo Metropolitan Government Building, the administrative center of the Tokyo Metropolitan Government. As of 2018, the ward has an estimated population of 346,235 and a population density of 18,232 people per km2. The total area is 18.23 km2. Since the end of World War II, Shinjuku has become a major secondary center of Tokyo (fukutoshin), rivaling the original city center in Marunouchi.

Special wards are a special form of municipalities in Japan under the 1947 Local Autonomy Law. They are city-level wards: primary subdivisions of a prefecture with municipal autonomy largely comparable to other forms of municipalities.