The Seoul Metropolitan Subway is a metropolitan railway system consisting of 23 rapid transit, light metro, commuter rail and people mover lines located in northwest South Korea. The system serves most of the Seoul Metropolitan Area including the Incheon metropolis and satellite cities in Gyeonggi province. Some regional lines in the network stretch out beyond the Seoul Metropolitan Area to rural areas in northern Chungnam province and western Gangwon Province, that lie over 100 km (62 mi) away from the capital.

The Seoul Capital Area or Gyeonggi region, is the metropolitan area of Seoul, Incheon, and Gyeonggi Province, located in north-west South Korea. Its population of 26 million is ranked as the fourth largest metropolitan area in the world. Its area is about 12,685 km2 (4,898 sq mi). It forms the cultural, commercial, financial, industrial, and residential center of South Korea. The largest city is Seoul, with a population of approximately 10 million people, followed by Incheon, with 3 million inhabitants.