The is a rapid transit line in Tokyo and Chiba Prefecture, Japan, operated by the municipal Toei Subway. The line runs between in Ichikawa, Chiba in the east and in the west. At Shinjuku, most trains continue as through services to on the Keià  New Line, with some services continuing to in Sagamihara, Kanagawa via the Keià  Line and the Keià  Sagamihara Line.
On maps and signboards, the line is shown in the color leaf green. Stations carry the letter "S" followed by a two-digit number inside a lime green circle.
In fiscal year 2023, the Shinjuku Line was Toei's third most profitable line, earning in surplus. It served 704,235 passengers on average per day, the second highest in the Toei network (after the Oedo Line).
Unlike all other Tokyo subway lines, which were built to or gauges, the Shinjuku Line was built with a track gauge of to allow through operations onto the Keià  network. The line was planned as Line 10 according to reports of a committee of the former Ministry of Transportation; thus the rarely used official name of the line is the .
According to the Tokyo Metropolitan Bureau of Transportation, as of June 2009 the Shinjuku Line was the third most crowded subway line in Tokyo, at its peak running at 181% capacity between Nishi-Ã Âjima and Sumiyoshi stations.
It is the only Toei line to run outside Tokyo, and one of only two Tokyo subway lines to run into Chiba Prefecture, the other being the Tokyo Metro Tozai Line. The Tokyo Metro Yà «rakuchà  Line, the Tokyo Metro Fukutoshin Line and the Tokyo Metro Tozai Line are the only other subway lines to run beyond Tokyo, with the shared northern terminus of the first two at Wakà Âshi Station in Saitama Prefecture, and the eastern terminus of the Tozai Line being at Nishi-Funabashi Station in Chiba Prefecture. However, all lines that have through services contain at least one route beyond Tokyo.
In April 1968, the Urban Transportation Council proposed Line 10 as a route running from Roka-koen via Shinjuku and , passing through Ichigaya, Jimbochà Â, Sudachà Â, and Hamachà Â, and terminating at Sumiyoshichà Â.
Following this recommendation, the Ministry of Construction issued Notice No. 3731 on December 28 of the same year, formally approving the construction of the ShinjukuâÂÂSumiyoshichà  section (12.5 km) as Line 10, officially designated the Tokyo Metropolitan Rapid Transit Line No. 10. On 20 May 1969, Notice No. 2430 extended the planned route by incorporating the Keio Line between Chà Âfu and Shinjuku and further extending the urban planning area to Higashi-à Âjima, the site of the lineâÂÂs depot, bringing the total planned length to 31.2 km.
On 1 May 1971, construction began near Morishita and Sumiyoshi.
The Toei Shinjuku Line is served by the following types of 10-car EMUs. Until 11 August 2022, the line was also served by 8-car trains.
a. Crowding levels defined by the Ministry of Land, Infrastructure, Transport and Tourism: