Horsham is a market town on the upper reaches of the River Arun on the fringe of the Weald in West Sussex, England. The town is 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.

The River Arun is a river in the English county of West Sussex. At 37 miles (60 km) long, it is the longest river entirely in Sussex and one of the longest starting in Sussex after the River Medway, River Wey and River Mole. From the series of small streams that form its source in the area of St Leonard's Forest in the Weald, the Arun flows westwards through Horsham to Nowhurst where it is joined by the North River. Turning to the south, it is joined by its main tributary, the western River Rother, and continues through a gap in the South Downs to Arundel to join the English Channel at Littlehampton. It is one of the faster flowing rivers in England, and is tidal as far inland as Pallingham Quay, 25.5 miles (41.0 km) upstream from the sea at Littlehampton. The Arun gives its name to the Arun local government district of West Sussex.