Watford Junction is a railway station serving the town of Watford in Hertfordshire, England. The station is on the West Coast Main Line (WCML), 17 miles 34 chains (28 km) from London Euston and the Abbey Line, a branch line to St Albans. It is also the northern terminus of the Lioness line of the London Overground, which operates via the Watford DC line into central London.

Journeys into London take between 16 and 52 minutes, depending on the service. Trains also run to Clapham Junction and East Croydon, via the West London line. The station is a major hub for local bus services and the connecting station for buses to Warner Bros. Studio Tour London – The Making of Harry Potter. The station is located north of a viaduct over the Colne Valley and immediately south of Watford Tunnel.

The first railway station to open in Watford was situated on the north side of St Albans Road, approximately 200 metres (220 yd) further up the line from the present-day station. This small, single-storey red-brick building was built in 1836–7, when the first section of the London and Birmingham Railway (L&BR) was opened between London and Boxmoor. The station provided first and second-class waiting rooms, a departure yard, a carriage shed and engine house. The platforms were situated in a deep cutting which was accessed via a staircase.

In its 21 years of operation, it also served as a station for royalty; in the short period when the Dowager Queen Adelaide was resident at Cassiobury House (c.1846-49), this station was remodelled to provide her with a royal waiting room, and it was also reportedly used by Queen Victoria and Prince Albert on a trip to visit Sir Robert Peel in November 1843, when they travelled by road from Windsor Castle to take a train from Watford to Tamworth.

The old station closed when it was replaced by a new, larger station, which opened on 5 May 1858. The new Watford Junction station was located south of St Albans Road in order to accommodate the newly constructed branch line to St Albans. The junction station was rebuilt in 1909, and was extensively redeveloped in the 1980s. The Grade-II-listed Old Station House still stands at 147A St Albans Road, a rare surviving example of architecture from the beginning of the railway age; today, the building is occupied by a second-hand car dealership.

In 1862, the Watford and Rickmansworth Railway opened a route from Watford to Rickmansworth (Church Street). Now mostly closed, this route began by running south and west to a more central station on Watford's High Street, which remains in use.

From 1846, the L&BR was absorbed into the London and North Western Railway (LNWR) and Watford Junction was now run by this large, ambitious company. Seeking to compete with local buses and trams, the LNWR built an additional suburban line from Euston to Watford in the early years of the 20th century, known as the Watford DC line. This veered away from the main line at Bushey to loop around Watford to pass through the High Street station. A second suburban branch line was also built from High Street west towards Croxley Green to serve new housing developments in that area. Both branches were later electrified as part of this improvement plan, on the same DC three-rail system. The Rickmansworth branch was connected to the Main Line, via two through platforms, with a junction to the north; these platforms have since been partly built over and their remaining southern sections form part of the present DC lines terminus. At one time, tube-style trains were used on the branches to counter the low voltage caused by the lack of a sub-station near Rickmansworth.

The Bakerloo line was extended to Watford Junction in 1917, giving a shared service north of Willesden Junction with the main line electric trains which served Euston and Broad Street stations. However, since 1982 the line north of Harrow & Wealdstone has only been served by what is now the London Overground service from Euston station; this service uses these DC lines for its all-stations local service.