The Keio New Line (京王新線, Keiō Shinsen) is a 3.6-kilometer (2.2 mi) link which connects Keio Corporation's Keiō Line from Sasazuka Station in Shibuya to Shinjuku Station with through service on to the Shinjuku Line of the Tokyo Metropolitan Bureau of Transportation.

Except for a short section just before Sasazuka Station, the entire line is underground.

However, since new train cars are being designed to be able to operate on above-ground and below-ground tracks there is no real issue with the differentiation.

Although there are four types of trains that travel along the Keiō New Line segment (local, rapid, semi express, and express), all trains within the Keiō New Line stop at every station.

With the completion of the last segment of the Toei Shinjuku Line, through service operations began on March 30, 1980.