Heathrow Express is an airport rail link between London Heathrow Airport and London Paddington. It is an open-access operator, formed in 1998, and is operated by Heathrow Express Operating Company, a wholly-owned subsidiary of Heathrow Airport Holdings.

Heathrow Express was planned as a joint venture between BAA and British Rail but was taken over fully by the former following the privatisation of British Rail. Construction began in 1993. The principal works were two 5-mile single-bore tunnels (including eight escape shafts) and underground stations at Heathrow Central and Terminal 4. Electrification of the Great Western Main Line (GWML) between Paddington and Airport Junction, where the new line diverged from the GWML, was also required. A flying junction known as Stockley Flyover was constructed to connect the tunnel to the GWML fast lines. Beginning in January 1998, an interim service called Heathrow FastTrain ran to a temporary station called Heathrow Junction, where a coach took passengers the rest of the way.

From 1999 to 2003, a check-in service was provided at Paddington, allowing Heathrow Express passengers to check-in and drop off their luggage prior to flights, which was similar to the service currently provided on Hong Kong Airport Express. Checked baggage was transported to the airport by using the luggage space at the westbound first carriage. This service was withdrawn due to low usage and high cost of operation.

In June 2005, Heathrow Express began jointly providing a new Heathrow Connect service, which saw a new twice-hourly stopping service on the same route between Paddington and Heathrow using Class 360 EMUs from the Siemens Desiro family. Heathrow Airport Holdings had provided the on-board staff through Heathrow Express as part of the contract. This continued until May 2018, which saw Heathrow Connect absorbed into TfL Rail ahead of the new Crossrail project, which will see Crossrail fully operate to Heathrow Airport through this takeover when it opens.

In August 2018, Great Western Railway took over the operation of Heathrow Express as part of a new management contract. Heathrow Airport continues to be responsible for commercial aspects of the service, including marketing, ticket pricing and revenue management, while GWR are now responsible for operations as well as the replacement of the Class 332 fleet in the form of modified Class 387 units from its own fleet