The Seoul Metropolitan Subway (Korean: 수도권 전철) is a metropolitan railway system consisting of 24 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 network consists of multiple systems that form a larger, coherent system. These being the Seoul Metro proper, consisting of Seoul Metro lines 1 through 9 and certain light rail lines, that serves Seoul city proper and its surroundings; Korail regional rail lines, which serve the greater metropolitan region and beyond; Incheon Metro lines, operated by Incheon Transit Corporation, that serve Incheon city proper; and miscellaneous light rail lines, such as Gimpo Goldline and Yongin Everline, that connect lower-density areas of their respective cities to the rest of the network. Most of the system is operated by three companies – Seoul Metro, Korail (Korea Railroad Corporation), and Incheon Metro – with the rest being operated by an assortment of local municipal corporations and private rail companies.

The first line of the system, Line 1, started construction in 1971 and began operations in 1974, with through-operation to Korail's suburban railways. As of 2022, the network has 331.5 km (206.0 mi) of track on lines 1–9 alone.

The majority of the rolling stock used in the system was built by Hyundai Rotem, South Korea's leading train manufacturer.

The first line of the Seoul Subway network started construction in 1971. The first section of subway was built using the cheaper cut and cover construction method. Initial lines relied heavily on Japanese technology, and subsequent lines (until the early-2000s) procured technological imports from Japan and the United Kingdom (in particular, GEC Traction equipment used on wide-width Lines 2, 3 and 4 rolling stock from the 1980s). For example, Line 1 opened in 1974 with through services joining surrounding Korail suburban railway lines influenced by the Tokyo subway. Today, many of the Seoul Metropolitan Subway's lines are operated by Korail, South Korea's national rail operator.

The subway has free WiFi accessible in all stations and trains. All stations have platform screen doors. These safety doors were completed by 2017, however many stations previously had metal barriers installed decades beforehand. The world's first virtual mart for smartphone users opened at Seolleung station in 2011.

All directional signs in the system are written in Korean using Hangul, as well as English and Katakana/Chinese characters for Japanese and Mandarin Chinese. However the maps on the walls are in Korean and English only. In the trains, there are in addition many LCD screens giving service announcements, upcoming stop names, YTN news, stock prices and animated shorts. There are also prerecorded voice announcements that give the upcoming station, any possible line transfer, and the exiting side in Korean, followed by English. At major stations, this is followed by Japanese, then Mandarin Chinese, as well.

Seoul Subway uses full-color LCD screens at all stations to display real-time subway arrival times, which are also available on apps for smartphones. Most trains have digital TV screens, and all of them have air conditioning and climate controlled seats installed that are automatically heated in the winter. In 2014, it became the world's first metro operator to use transparent displays for ads when it installed 48 transparent displays on major stations of Line 2 in Gangnam District. All lines use the T-money smart payment system using RFID and NFC technology for automatic payment by T-money smart cards, smartphones, or credit cards and one can transfer to any of the other line within the system for free.