The Delhi Metro is a rapid transit system serving Delhi and its satellite cities of Bahadurgarh, Faridabad, Ghaziabad, Gurgaon and Noida in the National Capital Region of India. By far the largest and busiest metro in India, and second oldest after the Kolkata Metro it is the world's 4th longest metro system and 16th largest by ridership. The network consists of eight colour-coded regular lines, running 317 kilometres (197 mi) serving 231 stations. The system has a mix of underground, at-grade, and elevated stations using both broad-gauge and standard-gauge. Delhi Metro operates over 2,700 trips daily, starting at around 05:00 and ending at 23:30 hrs. In 2016–17, the Delhi Metro had an average daily ridership of 2.76 million passengers and a cumulative ridership of 1 billion passengers yearly.

Construction started in 1998, and the first section, on the Red Line, opened in 2002. The development of the network was divided into phases, Phase I containing 3 lines was completed by 2006, and Phase II in 2011. Phase III is currently under construction, and is scheduled to be mostly complete by 2018.

Delhi Metro Rail Corporation Limited (DMRC), a company with equal equity participation from the Government of India and the Government of Delhi, built and operates the Delhi Metro. DMRC was certified by the United Nations in 2011 as the first metro rail and rail-based system in the world to get carbon credits for reducing greenhouse gas emissions, reducing carbon emission levels in the city by 630,000 tonnes every year.

Delhi Metro also interchanges with the separately operated Rapid Metro Gurgaon, with a shared ticketing system.