The Mass Transit Railway system, known locally by the initialism MTR, is a rapid transit system in Hong Kong and the territory's principal mode of railway transportation. Operated by the MTR Corporation (MTRCL), it consists of heavy rail, light rail and feeder bus services, centred around a 10-line rapid transit network, serving the urbanised areas of Hong Kong Island, Kowloon, and the New Territories. The system encompasses 245.3 km (152.4 mi) of railways, as of December 2022, with 179 stations—including 99 heavy rail stations, 68 light rail stops and 1 high-speed rail terminus.

Under the government's rail-led transport policy, the MTR system is a common mode of public transport in Hong Kong, with over five and a half million trips made on an average weekday consistently achieving a 99.9% punctuality rate on its arrivals and departures. As of 2018, the MTR holds a 49.3% share of the franchised public transport market, making it the most popular transport option in Hong Kong. The integration of the Octopus card fare-payment technology into the MTR system in September 1997 has further facilitated commuting.

During the 1960s, the government of Hong Kong saw a need to accommodate increasing road traffic as Hong Kong's economy grew rapidly. In 1966, British transport consultants Freeman, Fox, Wilbur Smith & Associates were appointed to study the transport system of Hong Kong. The study was based on the projection of the population of Hong Kong for 1986, estimated at 6,868,000. On 1 September 1967, the consultants submitted the Hong Kong Mass Transport Study to the government, which recommended the construction of a 40-mile (64 km) rapid transit rail system in Hong Kong. The study suggested that four rail lines be developed in six stages, with a completion date set between December 1973 and December 1984. Detailed locations of lines and stations were presented in the study. These four lines were the Kwun Tong line (from Mong Kok to Ma Yau Tong), Tsuen Wan line (from Admiralty to Tsuen Wan), Island line (from Kennedy to Chai Wan Central), and Shatin line (from Tsim Sha Tsui to Wo Liu Hang).

The study was submitted to the Legislative Council on 14 February 1968. The consultants received new data from the 1966 by-census on 6 March 1968. A short supplementary report was submitted on 22 March 1968 and amended in June 1968. The by-census indicated that the projected 1986 population was reduced by more than one million from the previous estimate to 5,647,000. The dramatic reduction affected town planning. The population distribution was largely different from the original study. The projected 1986 populations of Castle Peak New Town, Sha Tin New Town, and, to a lesser extent, Tsuen Wan New Town, were revised downwards, and the plan for a new town in Tseung Kwan O was shelved. In this updated scenario, the consultants reduced the scale of the recommended system. The supplementary report stated that the originally suggested four tracks between Admiralty station and Mong Kok station should be reduced to two, and only parts of the Island line, Tsuen Wan line, and Kwun Tong line should be constructed for the initial system. The other lines would be placed in the list of extensions. This report led to the final study in 1970.

In 1970, a revised system with four lines was laid out in the British consultants' new report, Hong Kong Mass Transit: Further Studies. The four lines were to be the Kwun Tong line, Tsuen Wan line, Island line, and East Kowloon line. The lines that were eventually constructed were somewhat different compared to those presented in this report and the Hong Kong Mass Transport Study.

In 1972, the Hong Kong government authorised construction of the Initial System, a 20-kilometre (12 mi) system that roughly translates to today's Kwun Tong line between Kwun Tong and Prince Edward, Tsuen Wan line between Mei Foo and Admiralty, and Island line between Sheung Wan and Admiralty. The Mass Transit Steering Committee, chaired by the Financial Secretary Philip Haddon-Cave, began negotiations with four major construction consortia in 1973. The government's intention was to tender the entire project, based on the British design, as a single tender at a fixed price. A consortium from Japan, led by Mitsubishi, submitted the only proposal within the government's $5-billion price ceiling. They signed an agreement to construct the system in early 1974, but in December of the same year, pulled out of the agreement for reasons stemming from fears of the oil crisis.

Several weeks later, in early 1975, the Mass Transit Steering Group was replaced by the Mass Transport Provisional Authority, which held more executive powers. It announced that the Initial System would be reduced to 15.6 kilometres (9.7 mi) and renamed the "Modified Initial System" (now part of the Kwun Tong and Tsuen Wan lines). Plans for a single contract were abandoned in favour of 25 engineering contracts and 10 electrical and mechanical contracts. On 7 May 1975 the Legislative Council passed legislation setting up the government-owned Mass Transit Railway Corporation (MTRC) to replace the Mass Transport Provisional Authority, the Mass Transit Railway Ordinance.

Construction of the system began on 11 November 1975. The northern section was completed on 30 September 1979 and was opened on 1 October 1979 by Governor Murray MacLehose. Trains on this route ran from Shek Kip Mei to Kwun Tong in Phase 1, Tsim Sha Tsui to Kwun Tong in Phase 2 in December 1979, and Chater to Kwun Tong in the last phase, initially in a four-car configuration. The first train drivers were trained on the London Underground. It was designed by a consortium of consultants led by Freeman Fox and Partners. On later extensions to the railway the stations were designed under the supervision of Roland Paoletti, the chief architect at MTR.