The Northern line is a London Underground line that runs from North London to South London. It is printed in black on the Tube map. The Northern line is unique on the Underground network in having two different routes through central London, two southern branches and two northern branches. Despite its name, it does not serve the northernmost stations on the Underground, though it does serve the southernmost station at Morden, the terminus of one of the two southern branches.

The line's northern termini, all in the London Borough of Barnet, are at Edgware and High Barnet; Mill Hill East is the terminus of a single-station branch line off the High Barnet branch. The two main northern branches run south to join at Camden Town where two routes, one via Charing Cross in the West End and the other via Bank in the City, continue to join at Kennington in Southwark. At Kennington, the line again divides into two branches, one to each of the southern termini at Morden, in the borough of Merton, and Battersea Power Station in Wandsworth.

For most of its length it is a deep tube line. The portion between Stockwell and Borough opened in 1890 and is the oldest section of deep-level tube line on the network. About 294 million passenger journeys were recorded in 2016/17 on the Northern line, making it the busiest on the Underground. It has 18 of the system's 31 stations south of the River Thames. There are 52 stations in total on the line, of which 38 have platforms below ground.

The line has a complicated history. The longtime arrangement of two main northern branches, two central branches and the southern unification reflects its genesis as three separate railways, combined in the 1920s and 1930s. An extension in the 1920s used a route originally planned by a fourth company. Abandoned plans from the 1920s to extend the line further southwards, and then northwards in the 1930s, would have incorporated parts of the routes of two further companies. From the 1930s to the 1970s, the tracks of a seventh company were also managed as a branch of the Northern line. An extension of the Charing Cross branch from Kennington to Battersea opened on 20 September 2021, giving the line a second southern branch. There are also proposals to split the line into separate lines following the opening of the new link to Battersea.