The Metro Tunnel, formerly known as Melbourne Metro Rail (MMR), is an underground suburban rail project currently under construction in Melbourne, the state capital of Victoria and second-largest city in Australia. It involves the construction of twin 9 km (5.6 mi) heavy commuter rail tunnels between South Yarra southeast of the Melbourne central business district (CBD) and South Kensington northwest of the CBD, with five new underground stations. The tunnel will connect the busy Pakenham/Cranbourne lines with the Sunbury line north-south through the center of the Hoddle Grid, creating a new high-frequency cross-city line that bypasses Flinders Street station and the City Loop. The Metro Tunnel is also planned to serve as a rail transit link to Melbourne Airport via a proposed new line branching off Sunshine called the Melbourne Airport Rail, which eventually will link up with the proposed Suburban Rail Loop orbital line.

The Metro Tunnel project is managed by the Victorian Infrastructure Delivery Authority and is part of the Victorian Government's "Big Build" infrastructure initiative, at an estimated cost of A$12.8 billion. The project aims to enable the operational separation of various existing lines on Melbourne's rail network and increase the capacity of the system to metro-style frequencies. The Metro Tunnel has high-capacity signalling and platform screen doors, both a first for Melbourne. In addition to relieving the peak-hour ridership pressure upon the City Loop, the tunnel will also bring rapid transit services to the University of Melbourne, various major tertiary hospitals and health science research institutes in Parkville (collectively known as the Melbourne Biomedical Precinct), and the Royal Botanic Gardens, all of which previously relied on trams and buses for last-mile transit to and from commuter railway stations.

The Andrews Government began planning the project in 2015. Initial construction works commenced in early 2017. Sections of the Melbourne CBD, including City Square and parts of Swanston Street, were closed to enable construction of the tunnel and stations. Tunnelling began in 2019 and was completed in 2021. Originally expected to be completed in 2026, the Metro Tunnel will open for certain services in early December 2025, with full services to commence on 1 February 2026.

Melbourne's original development occurred at a time when railway technology began to emerge as a feasible and efficient mode of transit. This led to a symbiotic relationship between the CBD and the rail network which grew to surround it. An almost purely radial system of lines, developed largely before 1930, linked the growing suburbs to the economic hub of the city centre, producing a system which supported the daily flow of passengers into and out of the city to access employment opportunities. Despite the increasingly car-oriented developments of the mid-20th century, the suburban rail lines in Melbourne continued to discourage any decentralisation of employment, leaving the city unusually dependent on its central core when compared to cities of similar size globally.

The first underground rail line to be built in Melbourne was the City Loop, which began construction in 1971 and opened gradually between 1981 and 1985. Among its aims were to reduce pressure on Flinders Street station by distributing passengers to three additional stations in the city centre (Parliament, Melbourne Central, Flagstaff), and to improve the capacity of the network's central core by eliminating the need for trains to change direction after terminating at Flinders Street. However, it was not entirely successful in achieving these aims. The four tunnels of the Loop proved to be a capacity constraint on the ten main railway lines entering the CBD, and the peculiarities of operating four single-direction balloon loops meant that inner-city rapid transit was difficult for passengers. At the same time, the Loop consumed much of the capital available for investment in the city's rail system. As a result, the extensions to the outer suburban network which had been envisaged as a succession to the Loop itself did not eventuate. Meanwhile, patronage on the network had entered a long period of decline, which culminated in the Lonie Report of 1980 recommending the closure of several lines.

The need for an overhaul of the existing commuter rail network was first discussed in the early 2000s as unprecedented population growth began to place significant pressure on existing rail infrastructure and constraints on the inner core of the network as it approached capacity. Other problems faced by the network in the first decade of the 21st century included inefficient operations which had developed during years of low patronage, and a loss of corporate memory, caused in part by the privatisation of rail services in the late 1990s, which limited the flexibility of planners in dealing with the burgeoning passenger numbers. Consequently, a large number of services were experiencing major overcrowding in peak periods. A series of planning documents released during the early 2000s, including Melbourne 2030 (2002), Linking Melbourne (2004) and Meeting Our Transport Challenges (2006) identified that significant capacity constraints existed in the central core and on the Dandenong corridor, but did not propose any significant capital works in the city centre, instead suggesting that the issues could be resolved by relatively minor operational changes and construction of a third track to Dandenong.

Outside the state government, support grew for a more substantial augmentation of the rail network, with many such ideas based on new underground lines through the CBD. In 2005, The Age reported that it had received a number of proposals from planning experts and engineers for rail "loops and arcs" in the central city, and publicised a plan published by Monash University professor Graham Currie for a tunnel between the University of Melbourne to the north of the city and South Yarra station to the south-east. Currie's plan also envisaged extensive improvements to the Melbourne tram network, including upgrading lines along St Kilda Road and Chapel Street to light rail standards. In 2006, the state government considered a plan to construct a combined road and rail tunnel beneath the Yarra River to provide an alternative to the West Gate Bridge, but the idea was deemed unfeasible.

By 2007, the planned third track to Dandenong was effectively abandoned, with no money provided for the project in that year's state budget, and opposition growing from the Public Transport Users Association and others. Later that year, it emerged that train operator Connex and coordinating authority Metlink were among stakeholders encouraging the government to consider a proposal similar to Currie's, but extended to Footscray in the city's west. Melbourne City Council, on the other hand, proposed a tunnel conceptually similar to the Currie plan, but running from Jewell station in the north to Windsor in the south-east.