Seoul Subway Line 2 (dubbed The Green Line but also The Circle Line) is a circular line of the Seoul Metropolitan Subway. The line running clockwise is called the "inner circle line" and the counter-clockwise line is called the "outer circle line." This is Seoul's most heavily used line, and the longest circular subway line in the world totaling 60.2 km.

Headways on the line vary from 2 minutes 18 seconds on peak periods and 5-6 minutes off-peak periods. The line connects the city centre to Gangnam, Teheran Valley and the COEX/KWTC complex.

In December 2010 the line is recorded as having the highest WiFi data consumption in the Seoul Metropolitan area. It averaged 2.56 times more than the other 14 subway lines fitted with WiFi service zones.

Line 2 was built in 1978-84 together with the Seongsu Branch (the second Sinjeong Branch was built 1989-95). Dangsan bridge was closed for reconstruction in 1996 and reopened on 22 November 1999. The old steel girder bridge was replaced by a 1.3-kilometre (0.81 mi) long concrete bridge between Dangsan on the southern side of the river and Hapjeong on the northern bank.

The Seoul Metropolitan Subway or Metropolitan Subway in Seoul, in Seoul, South Korea, is one of the most heavily used rapid transit systems in the world, which services 7 million passengers by the metropolitan system alone and well over 8 million trips daily by the greater system's sixteen lines consisted of the Seoul Metro, Seoul Metropolitan Rapid Transit Corporation & Korail commuter lines. The system serves Seoul, Incheon, Gyeonggi-do, northern Chungcheongnam-do, and western Gangwon-do. There are connections to the Incheon International Airport Railroad (A'REX) at Seoul Station and Gimpo Airport, and a free transfer with the single-line Incheon Subway system at Bupyeong on Seoul Subway Line 1. Since 2008, all stations operated by Seoul Metro and SMRT have been fitted with Platform Screen Doors, but only within Seoul and excluding stations operated by Korail.

All directional signs in the Seoul Subway are written in both Korean and English. Station signs and some maps also display Hanja. The pre-recorded voice announcement in the trains indicating the upcoming station, possible line transfer and exiting side are all in Korean followed by English, and in some cases Japanese then Chinese.

Seoul (Korean pronunciation: [sʌ.ul] ( listen), English /soʊl/), officially the Seoul Special City, is the capital and largest metropolis of South Korea. A megacity with a population of over 10 million, it is the largest city proper in the OECD developed world. The Seoul National Capital Area is the world's second largest metropolitan area with over 25 million inhabitants, which includes the surrounding Incheon metropolis and Gyeonggi province. Almost a quarter of South Koreans live in Seoul, half of South Koreans live in the metropolitan area, along with over 275,000 international residents.

Located in the Han River, Seoul has been a major settlement for over 2,000 years, with its foundation dating back to 18 B.C. when Baekje, one of the Three Kingdoms of Korea, established its capital in what is now south-east Seoul. It continued as the capital of Korea during the Joseon Dynasty and the Korean Empire. The Seoul National Capital Area is home to four UNESCO World Heritage Sites: Changdeokgung, Hwaseong Fortress, Jongmyo Shrine and the Royal Tombs of the Joseon Dynasty.

Line 2 or 2 Line can refer to: