The Piccadilly line is a deep-level London Underground line running from the north to the west of London. It has two branches, which split at Acton Town, and serves 53 stations. The line serves Heathrow Airport, and some of its stations are near tourist attractions such as Piccadilly Circus and Buckingham Palace. The District and Metropolitan lines share some sections of track with the Piccadilly line. Printed in dark blue (officially "Corporate Blue", Pantone 072) on the Tube map, it is the fourth busiest line on the Underground network, with over 210 million passenger journeys in 2011/12.

The first section, between Finsbury Park and Hammersmith, was opened in 1906 as the Great Northern, Piccadilly and Brompton Railway (GNP&BR). The station tunnels and buildings were designed by Leslie Green, featuring ox-blood terracotta facades with semi-circular windows on the first floor. When Underground Electric Railways of London (UERL) took over the line, it was renamed the Piccadilly line. Subsequent extensions were made to Cockfosters, Hounslow West and Uxbridge in the early 1930s, when many existing stations on the Uxbridge and Hounslow branches were rebuilt to designs by Charles Holden of the Adams, Holden & Pearson architectural practice. These were generally rectangular, with brick bases and large tiled windows, topped with a concrete slab roof. The western extensions took over certain existing District line services, which were fully withdrawn in 1964.

Stations in central London were rebuilt to cater for a higher volume of passenger traffic. To prepare for World War II, some stations were equipped with shelters and basic amenities, and others with blast walls. Construction of the Victoria line, the first section of which opened in 1968, helped to relieve congestion on the Piccadilly line; some sections of the Piccadilly had to be rerouted for cross-platform interchange with the new line. Several plans were made to extend the Piccadilly line to serve Heathrow Airport. The earliest approval was given in 1967, and the Heathrow extension opened in stages between 1975 and 1977. This served only Terminals 2 and 3 and the former Terminal 1. The line was extended again twice, to Terminal 4 via a loop in 1986, and to Terminal 5 directly from the main terminal station in 2008.

This line has two depots, at Northfields and Cockfosters, with a group of sidings at several locations. There are crossovers at a number of locations, some of which allow trains to switch to different lines. The Piccadilly line's electric power was formerly generated at Lots Road Power Station. This was taken out of use in 2003, and the line is now powered from the National Grid network. 1973 Stock trains are used on the line, 78 of which are needed to operate a 24 trains per hour (tph) service (a train every 2+1⁄2 minutes) during peak hours. These trains are due to be replaced by New Tube for London (NTfL) trains in the 2020s.