Shinjuku Nishiguchi Station is essentially a satellite station to Shinjuku. Its location is at the North-West area of Shinjuku Station. The name "Nishiguchi" is translated into "West Exit" which is where this station's exits are located, relative to Shinjuku Station. While this station is close to the Oedo line's Shinjuku station, this station is closer to the Tokyo Metro Marunouchi Line, the Seibu Shinjuku Line, and Shinjuku's bus terminal.
Like most stations on the Oedo line, this station has a platform that runs between both trains. There are two major exits in this station. The North ticket gate leads to various exit points to Oogado Crossing. The Southern gate leads to the Odakyu Department store area and the Marunouchi line.