Line 12, also known as MetroSur (Spanish for "MetroSouth"), is a line of the Madrid Metro inaugurated on 11 April 2003. Line 12 is a circle line that is not in the city of Madrid, but links five suburban towns southwest of the city, serving around one million people. The towns linked by Line 12 are Alcorcón, Leganés, Getafe, Fuenlabrada and Móstoles. Despite parts of the line running through uninhabited territory, there are no above-ground sections in an effort to facilitate future urban development. At a total length of 40.6 km (25.2 mi), it is the longest line in the network and even more extensive than Lines 9 and 10 with their appendages to the north and south. Despite this, Line 12 accumulated only about 32 million trips in 2018, down from 34.8 million in the previous year, making it the only line to lose users in that time and the third least used after the yet-to-be-completed Line 11 and the airport connection Line 8. This decrease was mainly caused by renovation works necessitating the temporary closure of sections of the line.

The Madrid Metro is a rapid transit system serving the city of Madrid, capital of Spain. The system is the 14th longest rapid transit system in the world, with a total length of 293 km (182 mi). Its growth between 1995 and 2007 put it among the fastest-growing networks in the world at the time. However, the European debt crisis greatly slowed expansion plans, with many projects being postponed and canceled. Unlike normal Spanish road and rail traffic, which drive on the right, Madrid Metro trains use left-hand running on all lines because traffic in Madrid drove on the left until 1924, five years after the system started operating.