The Buenos Aires Underground (Spanish: Subterráneo de Buenos Aires), locally known as Subte (Spanish pronunciation: [ˈsuβte], from subterráneo – 'underground' or 'subterranean'), is a rapid transit system that serves the area of the city of Buenos Aires, Argentina. The first section of this network (Plaza de Mayo-Plaza Miserere) opened in 1913, making it the 13th subway in the world and the first underground railway in Latin America, the Southern Hemisphere, and the Spanish-speaking world, with the Madrid Metro opening five years later, in 1919. Currently, Buenos Aires is the only Argentine city with a metro system.

Currently, the underground network's six lines—A, B, C, D, E, and H—comprise 56.7 kilometers (35.2 mi) of route that serve 90 stations. The network is complemented by the 7.4-kilometre-long (4.6 mi) Premetro line, and the 26-kilometre-long (16 mi) Urquiza suburban line, with 17 more stations in total. Traffic on lines moves on the left because Argentina drove on the left at the time the system opened. Over a million passengers use the network, which also provides connections with the city's extensive commuter rail and bus rapid transport networks.

The network expanded rapidly during the early decades of the 20th century; by 1944, its main routes were completed, with the addition of its newest line occurring as late as 2007. The pace of expansion fell sharply after the Second World War. In the late 1990s, expansion resumed at a quicker pace, and four new lines were planned for the network. Despite this, the network's expansion has been largely exceeded by the transportation needs of the city and is said to be overcrowded. As of 2015, two modernisation plans have been presented: City of Buenos Aires law 670, proposing the creation of 3 new lines (F, G, and I), and the PETERS plan, wherein 2 lines are created and the I line is postponed for future expansion, plus several other route amendments. Since 2019, there are no expansions under construction, for the first time in half a century.The entire network was nationalised in 1939, remaining in state hands and operation until the mid-1990s, when it entered into a concession model. The previously state-operated lines were offered as 20-year concessions to interested private parties; the two complementary lines were also included in this privatisation, and all have been operated by Metrovías since 1995, though the network and rolling stock remain the property of the City of Buenos Aires.