The Bakerloo line () is a London Underground line that goes from Harrow & Wealdstone in suburban north-west London to Elephant & Castle in south London, via the West End. Printed in brown on the Tube map, it serves 25 stations, 15 of which are underground, over 14.4 miles (23.2 km). It runs partly on the surface and partly in deep-level tube tunnels.

The line's name is a portmanteau of its original name, the Baker Street & Waterloo Railway. From Queen's Park to Harrow & Wealdstone (the section above ground), the line shares tracks with the London Overground Watford DC line and runs parallel to the West Coast Main Line. There is, however, a short tunnel at the western end of Kensal Green.

Opened between 1906 and 1915, many of its stations retain elements of their design to a common standard: the stations below ground using Art Nouveau decorative tiling by Leslie Green, and the above-ground stations built in red brick with stone detailing in an Arts & Crafts style. It is the ninth-busiest line on the network, carrying more than 111 million passengers annually.

The line uses 1972 Stock trains, which are the oldest trains in passenger service in Britain at 51 years old.