The Airport line is a suburban railway line in Perth, Western Australia, which is operated by the Public Transport Authority as part of the Transperth system. The Airport line is a branch of the Midland line and runs underground between Bayswater and High Wycombe via Perth Airport. The branch is 8.6 kilometres (5.3 mi) long and has three stations: Redcliffe, Airport Central, and High Wycombe stations. Airport line services continue west of Bayswater along the Midland and Fremantle lines via Perth station to terminate at Claremont station.

Known during construction as the Forrestfield–Airport Link, the Airport line was constructed by a joint venture between Salini Impregilo and NRW for A$1.86 billion. Construction started in November 2016 and tunnel boring started in July 2017. The construction process was beset by several problems, including ground disturbances caused by tunnelling, a sinkhole and groundwater leak during the construction of a cross-passage, several worker injuries, and poor work conditions. Tunnelling was completed in April 2020, which was followed by the installation of overhead line equipment, tracks, and other equipment. In addition to the Forrestfield–Airport Link, works included a turnback siding at Claremont station for Airport line trains to turn around, and upgrades to Claremont and Bayswater stations. Originally planned to open in 2020, the project was delayed several times due to the aforementioned problems and later supply chain issues caused by the COVID-19 pandemic. The line opened on 9 October 2022, over two years late and $20 million under budget.

Transperth B-series trains, three cars in length, operate on the Airport line every 12 minutes during peak and every 15 minutes outside peak and on weekends. All Airport line branch stations are fully accessible and have 150-metre (490 ft) long platforms, long enough for a six-car train. Train lengths are limited by most Midland and Fremantle line stations, which have platforms only 100 metres (330 ft) long. The installation of communications-based train control by 2027 will allow frequencies to increase and planned platform lengthening will allow train lengths to increase. The line was forecast to have 20,000 boardings per day upon opening, but six months after it opened, the Airport line has 12,000 boardings per day. In the 2024–25 financial year, the Airport line had 5,619,693 boardings.

The 2004 Perth Airport master plan proposed a light rail spur off the Midland line between Bayswater and Ashfield stations, travelling above ground parallel to Tonkin Highway and along Brearley Avenue to the domestic terminal (terminals 3 and 4) and underground to the international terminal (terminals 1 and 2). The Midland line between Bayswater and Perth would have been a shared light rail and heavy rail corridor, and there would have been new stations at Great Eastern Highway, the domestic terminal, and the international terminal. The Labor state government in the 2000s started planning for this, having applied to Infrastructure Australia for federal funding.

Following the 2008 state election, the new Liberal government withdrew the application to Infrastructure Australia, with Premier Colin Barnett saying that he believed airport rail links were not viable in cities the size and density of Perth. The Public Transport Authority (PTA) nonetheless continued planning for the rail extension to the airport. Planning focused on building the line above ground between the Midland line at Bayswater and the Airport alongside Tonkin Highway, before tunnelling under the airport and emerging on the eastern side at High Wycombe.

In September 2012, the government announced the half tunnelled, half above ground heavy rail line as its preferred route and said it was looking at costing and when the Airport line would be built. In December 2012, the Labor opposition announced its Metronet plan to rapidly expand Perth's rail network, which it would take to the 2013 state election in March. This plan had the Airport line as part of a loop line which would connect to the Armadale line, Mandurah line and Fremantle line to the south and the Midland line to the north. The opposition revealed further details of its proposed route, making it follow a 10.5-kilometre (6.5 mi) fully above-ground route, requiring going around the airport's runways. This would have made the Airport station far away from the international terminal, requiring a 1 kilometre (1 mi) shuttle bus between the station and terminal. The Labor Party costed the route at A$731.5 million, but Treasury costed the route at $1.446 billion.

In February 2013, the government committed to its own plans of building the Airport line by 2018 at a cost of $1.895 billion. The 2013 election resulted in the government's re-election, so the half tunnelled route entered further planning. By the end of 2013, Transport Minister Troy Buswell said he was considering making the Airport line entirely underground from Bayswater to High Wycombe as it could be better value for money. Transport expert Peter Newman and Opposition Leader Mark McGowan criticised the idea of building the line entirely underground, saying it would be far more expensive.

Cabinet approved the project, now known as the Forrestfield–Airport Link, in August 2014. The Forrestfield–Airport Link involved building twin-bored 8.5-kilometre (5.3 mi) tunnels from the Midland line at Bayswater to High Wycombe, passing under the Swan River and Perth Airport. There were three stations along the route: Airport West near terminals 3 and 4, Consolidated Airport near terminals 1 and 2, and Forrestfield. Airport West and Forrestfield stations were planned to have bus interchanges. Airport West and Consolidated Airport stations were planned to be underground, and Forrestfield station was planned to be above ground. It was forecast the link would have 20,000 boardings per day upon opening, increasing to 29,000 by 2031. The Airport line service was planned to interline with the Midland line west of Bayswater, passing through Perth station before interlining with the Fremantle line to Daglish station, where Airport line trains would terminate and turn around. It was estimated to cost $2.2 billion, approximately $300 million more than promised in 2013, and open in 2020, two years later than promised.