Horsham (/ˈhɔːrʃəm/) is a market town on the upper reaches of the River Arun on the fringe of the Weald, in West Sussex, England. The town lies 31 miles (50 km) south-south-west of London, 18.5 miles (30 km) north-west of Brighton and 26 miles (42 km) north-east of the county town of Chichester. Nearby towns include Crawley to the north-east and Haywards Heath and Burgess Hill to the south-east. It is the administrative centre of the Horsham district.

There are two main tiers of local government covering Horsham, at district and county level: Horsham District Council and West Sussex County Council. Much of the built-up area of Horsham is an unparished area, but some of the suburbs are included in civil parishes, notably North Horsham.

The town is the centre of the parliamentary constituency of Horsham, recreated in 1983. Jeremy Quin had served as Conservative Member of Parliament for Horsham since 2015, succeeding Francis Maude, who held the seat from 1997 but retired at the 2015 general election. Quin was defeated in the 2024 election. John Milne is the town's current MP.

Horsham was an ancient parish in the Hundred of Singlecross in the Rape of Bramber. The parish covered the town and surrounding rural areas including Broadbridge Heath, Roffey and Southwater.

The town was an ancient borough, being described as a borough from the thirteenth century and also electing members of parliament from 1295 onwards. By the eighteenth century the borough corporation had ceased to have much role in administering the town, instead serving primarily as the means by which the main landowners, the Dukes of Norfolk, controlled the election of MPs. Dealing with such rotten boroughs was part of the motivation behind the Reform Act 1832, which reduced Horsham's representation from two to one MPs and made elections less open to abuse. Following those reforms Horsham's borough corporation stopped functioning.

Local government eventually returned to Horsham in 1875 when the central part of the parish, containing the town, was made a local government district, governed by a local board. Such districts were reconstituted as urban districts under the Local Government Act 1894. The 1894 Act also said that parishes could no longer straddle district boundaries, and so the part of Horsham parish outside the urban district became a separate parish called Horsham Rural.

Horsham Urban District was abolished in 1974, merging with surrounding districts to become the modern Horsham District, which covers a large rural area as well as the town itself. The Horsham Rural parish continued to exist until 1987 when it was divided into the parishes of Broadbridge Heath, North Horsham and Southwater, subject to some adjustments to the boundaries with neighbouring parishes at the same time.

Horsham lies 50 metres (160 ft) above sea level. It is in the centre of the Weald in the Low Weald, at the western edge of the High Weald, with the Surrey Hills of the North Downs to the north and the Sussex Downs of the South Downs National Park to the south. The River Arun rising from ghylls in the St Leonard's Forest area, to the east of Horsham, cuts through the south of the town then makes its way through Broadbridge Heath. The Arun is joined by a number of streams flowing down from the north, which rises around Rusper.