It has been argued that the area was a bridgehead for the Roman invasion of Britain. The city centre stands on the foundations of the Romano-British city of Noviomagus Reginorum, capital of the Civitas Reginorum, and near to the Roman Palace of Fishbourne. 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 Roman road of Stane Street, connecting the city with London, started at the east gate, while the Chichester to Silchester road started from the north gate. The city streets have a cross-shaped layout, inherited from the Romans: radiating outwards from the medieval market cross lead the North, South, East and West shopping streets.

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 close to what would have been the city walls, outside the East Gate in around 80 AD. The remains are now buried under land currently used as a park, but the bank of the amphitheatre is clearly discernible and a notice board in the park gives more information.

When the Domesday Book was compiled, Chichester 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 Earl Roger de Mongomerie for courageous efforts in the battle.