The area around Chichester is believed to have played significant part during the Roman Invasion of A.D 43, as confirmed by evidence of military storage structures in the area of the nearby Fishbourne Roman Palace.The city centre stands on the foundations of the Romano-British city of Noviomagus Reginorum, capital of the Civitas Reginorum. The Roman road of Stane Street, connecting the city with London, started at the east gate, while the Chichester city to Silchester road started from the north gate. The plan of the city is inherited from the Romans: the North, South, East and West shopping streets radiate from the central market cross dating from medieval times.

The original Roman city wall was over two metres thick with a steep ditch (which was later used to divert the River Lavant). It survived for over one and a half thousand years but was then replaced by a thinner Georgian wall.An amphitheatre was built outside the city walls, close to the East Gate, in around 80 AD. The area is now a park, but the site of the amphitheatre is discernible as a gentle bank approximately oval in shape; a notice board in the park gives more information.

According to the Anglo-Saxon Chronicle it was captured towards the close of the fifth century, by Ælle, and renamed after his son, Cissa. It was the chief city of the Kingdom of Sussex.The cathedral for the South Saxons was founded in 681 at Selsey; the seat of the bishopric was moved to Chichester city in 1075.Chichester was one of the burhs established by Alfred the Great, probably in 878-9, making use of the remaining Roman walls. According to the Burghal Hidage, a list written in the early 10th century, it was one of the biggest of Alfred's burhs, supported by 1500 hides, units of land required to supply one soldier each for the garrison in time of emergency. The system was supported by a communication network based on hilltop beacons to provide early warning. It has been suggested that one such link ran from Chichester to London.

When the Domesday Book was compiled, Chichester city consisted of 300 dwellings which held a population of 1,500 people. There was also a mill named Kings Mill that would have been rented to local slaves and villeins. After the Battle of Hastings the township of Chichester was handed to Roger de Mongomerie, 1st Earl of Shrewsbury, for courageous efforts in the battle, but it was forfeited in 1104 by the 3rd Earl. Shortly after 1066 Chichester Castle was built by Roger de Mongomerie to consolidate Norman power.In around 1143 the title Earl of Arundel was created and became the dominant local landowner. Between 1250 and 1262, the Rape of Chichester city was created from the western half of Arundel rape, with the castle as its administrative centre.