The Osaka Metro Midōsuji Line is a rapid transit line in Osaka, Japan, operated by Osaka Metro. Constructed under Midōsuji, a major north-south street, it is the oldest line in the Osaka subway system and the second oldest in Japan, following the Tokyo Metro Ginza Line. Its official name is Rapid Electric Tramway Line No. 1 (高速電気軌道第1号線), while the Osaka Municipal Transportation Bureau refers to it as Osaka City Rapid Railway Line No. 1 (大阪市高速鉄道第1号線), and in MLIT publications it is referred to as Line No. 1 . On line maps, stations on the Midōsuji Line are indicated with the letter "M".

The Osaka Metro is a major rapid transit system in the Osaka Metropolitan Area of Japan, operated by the Osaka Metro Company, Ltd. It serves the city of Osaka and the adjacent municipalities of Higashiosaka, Kadoma, Moriguchi, Sakai, Suita, and Yao. Osaka Metro forms an integral part of the extensive mass transit system of Greater Osaka, having 123 out of the 1,108 rail stations (2007) in the Osaka-Kobe-Kyoto region. In 2010, the greater Osaka region had 13 million rail passengers daily of which the Osaka Municipal Subway accounted for 2.29 million.