The Northern line is a London Underground line that runs from north 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.

In 2021 the line was extended. This is known as the The Northern Line Extension (NLE).

The Northern line is serviced by four depots. The main one is at Golders Green, adjacent to Golders Green tube station, while the second, at Morden, is south of Morden tube station and is the larger of the two. The other two are at Edgware and Highgate. The Highgate depot is on the former LNER branch to Alexandra Palace. There was originally a depot at Stockwell but it closed in 1915. There are sidings at High Barnet for stabling trains overnight.

This serves 52 stations and is 36 miles long.