The Chennai Metro is a rapid transit system serving the city of Chennai, India. Operated by Chennai Metro Rail Limited (CMRL) – a joint venture between the Government of India and the Government of Tamil Nadu, the network consists of two lines and spans 54.1 km (33.6 mi). The system uses standard gauge and has 41 underground and elevated stations.

Planning for the metro started in 2007–08 with construction commencing in February 2009. After testing in 2014, the first segment of the metro between the Alandur and Koyambedu stations on the Green Line began operations on 29 June 2015. On 21 September 2016, operations on the Blue Line commenced between Chennai Airport and Little Mount. The underground stretch from AG-DMS to Washermanpet of Blue Line opened on 10 February 2019, completing phase 1 of the Metro.

Three more lines covering a length of 118.9 km (73.9 mi) are under construction as part of the second phase. The Chennai Mass Rapid Transit System (MRTS), operated by Indian Railways, is planned to be incorporated into the Chennai Metro. A light rail has been proposed to connect Tambaram on the Chennai suburban rail network with Velachery on the MRTS. Further extensions are also planned for the three lines under construction.

A single electrified line from Chennai Beach to Tambaram began operations in 1931 as part of the Chennai Suburban Railway. Two other rail lines were subsequently built, connecting Chennai Central to Gummidipoondi and Tiruvallur in 1979. In 1965, the Planning Commission evaluated the capacity of the transportation network in major metropolitan cities including Madras, and developed plans for further expansion of transportation amenities. As a result, an initial stretch of the Chennai Mass Rapid Transit System (MRTS) between Chennai Beach and Chepauk opened in 1995 – the first elevated rail line in India. The MRTS later extended to Thirumayilai in 1997, to Thiruvanmiyur in 2004, and to Velachery in 2007. In 2006, a modern metro rail system for Chennai was planned, to be modeled after the Delhi Metro.

The Delhi Metro Rail Corporation (DMRC) was tasked to prepare a project report on the implementation of metro system in Chennai. The Government of Tamil Nadu approved the first phase of the project in November 2007. The first phase was planned with two lines covering 45.1 km (28.0 mi) with 25 km (16 mi) being underground. The first corridor would connect Washermanpet with Chennai International Airport extending for 23.1 km (14.4 mi) with 14.3 km (8.9 mi) being underground and the second corridor would connect St.Thomas Mount with Chennai Central extending for 22 km (14 mi) with 9.7 km (6.0 mi) being underground. In December 2007, the Chennai Metro Rail Limited (CMRL) was established as a joint venture between the Governments of India and Tamil Nadu to execute the project. The Planning commission gave in-principle approval for the project in April 2008 with the final approval by the Union Cabinet of Government of India in January 2009. The estimated base cost of the first phase was ₹146 billion (US$1.7 billion) of which 59% was loaned by Japan International Cooperation Agency (JICA). The Government of India contributed 15% of the cost with the Government of Tamil Nadu financed the remaining 21% of the project.

In February 2009, a ₹1.99 billion (US$24 million) contract was awarded for the construction of a 4.5 km (2.8 mi) long viaduct along the Inner Ring Road between Koyambedu and Ashok Nagar and construction started on 10 June. In March 2009, a US$30 million contract was awarded to a consortium of five companies headed by French company Egis for technical consultancy on the project. In August 2010, the contract for supplying rolling stock was awarded to Alstom for US$243 million. In January 2011, a joint venture between Larsen and Toubro (L&T) and Alstom was awarded the contract for design and construction of track works and a depot at Koyambedu for ₹4.49 billion (US$53 million).

Contracts for the construction of underground sections of the first phase were awarded to Gammon India and Mosmetrostroy in February 2011. Subsequently in June, tenders for the elevated stations of the first phase were awarded to Consolidated Construction Consortium. The contract for supply of lifts and escalators was awarded to a joint venture of Johnson Lifts and SJEC Corporation. The contract for power supply and overhead electrification was awarded to Siemens for ₹3.05 billion (US$36 million) and contracts for automatic fare collection (AFC), tunnel ventilation and air conditioning were awarded to Nippon Signal, Emirates Trading Agency and Voltas respectively. In July 2012, the first tunnel boring machine was launched and by October that year, eleven machines were commissioned to bore tunnels by three consortia – Afcons-Transtonnelstroy, L&T and SUCG.

In August 2012, a construction worker was killed with six others seriously injured due to a crane failure near Pachaiyappa's College and on 10 January 2013, a 22-year-old construction worker was killed with three others injured at a construction site between Alandur and St Thomas Mount. On 6 November 2013, a test run was conducted along a stretch of 1 km (0.62 mi) track. The project faced several delays and missed deadlines due to problems with land acquisition and cancellation of construction contracts. On 11 January 2014, a crane toppled over, killing a 20-year-old construction worker and seriously injuring another at Saidapet.