Willesden Junction (/ˈwɪlzdən ˈdʒʌŋkʃən/) is an interchange station located in Harlesden, north-west London. It is situated on the Bakerloo line of the London Underground and the Lioness and Mildmay lines of the London Overground. The station is located close to the Old Oak Lane conservation area in the East Acton ward.

The station developed on three contiguous sites: the West Coast Main Line (WCML) station was opened by the London & North Western Railway on 1 September 1866 to replace the London and Birmingham Railway's Willesden station of 1841 which was 0.5 miles (0.8 km) to the northwest. Passenger services ended in 1962 when the platforms were removed during the electrification of the WCML to allow the curvature of the tracks to be eased. Later the bridges for the North London line (NLL) were rebuilt.

The High-Level station on the NLL was opened by the North London Railway in 1869 for two Richmond tracks and later for two Shepherds Bush tracks, both crossing the WCML roughly at right angles. In 1894 a new, combined High-Level station was built, with an island platform plus a third shorter platform for Earls Court trains (which was later removed) together with a new station entrance building which still survives. By 1897 199 passenger trains and 47 goods trains passed through the High-Level station each day. The 'Willesden New Station' or Low-Level station on the Watford DC line was opened in 1910 to the north of the main line with two outer through platforms and two inner bay platforms at the London end. The bay platforms were originally long enough for four-coach Bakerloo trains when such trains ran outside peak times, but were shortened in the 1960s when a new toilet block was installed; in more recent times the platform buildings have been reconstructed and the bay length increased due to the addition of a fourth and then a fifth coach to class 378 trains.

In 1896 staff totalled 271, including 79 porters, 58 signalmen (in 14 signal boxes) and 58 shunters and yard foremen. They issued 1,006,886 tickets to passengers in 1896, up from 530,300 in 1886. Many of them were housed in what is now the Old Oak Lane conservation area, built by the LNWR in 1889 and which included an Institute, reading room and church.

The main-line platforms were numbered from the south side (including one or two on the Kensington route) followed by the high level platforms and then the DC line platforms which thus had the highest numbers. Later the surviving platforms were renumbered.

A freight liner terminal was opened in August 1967. It was built on an 18-acre site of the steam locomotive depot alongside the main electrified rail-link. It was opened by John Morris, Parliamentary Secretary for the Ministry of Transport. The terminal had the capacity to handle 2,000 containers a week.

In the late nineteenth century, it was nicknamed "Bewildering Junction" or "The Wilderness" because it contained such a maze of entrances, passages and platforms.

On 5 December 1910, a passenger train was in a rear-end collision with another at the station. Five people were killed and more than 40 were injured. The accident was due to a signaling error. In June 1936, one first-class passenger was killed and four other passengers were injured on the main line track just north of the station. An LMS passenger train heading to Watford was struck by a piece of equipment sticking out of the milk train heading to Euston. On 6 October 1986 at 17:00 a class 313 train collided with the rear of a stationary Bakerloo line train on the up line to the east of the station between the Scrubbs Lane overbridge and Kensal Green tunnel (the location was officially described as "Kensal Green"). 23 of 25 passengers were injured, all but one were discharged from hospital during the same evening.