Most of the city of Portsmouth lies on Portsea Island (or Landport which is often shown on records) which is where the Solent joins the English Channel, the island is separated from the mainland by a narrow creek in the north, which has three bridges to make it look like a peninsula. To the west of the island is Portsmouth Harbour and the large tidal bay of Langstone Harbour is to the east. The skyline to the north is dominated by the 120metre high chalk escarpment known as Portsdown Hill . The name of Portsea refers to the island and to that part of the city around HM Dockyards.

Originally Portsmouth was a fishing village on the south western tip of the island and had a dockyard even in the reign of King John and it was here that Henry VIII founded the world's very first dry dock back in 1495. During WWII a lot of the region was completely flattened by air raids but today it is a huge thriving community with a large housing estate replacing the Victorian housing. The dockyard was expanded during the 18th century and the town began to flourish the original Old Portsmouth and the area at the mouth of the Harbour was founded in the 12th cneutry and for many centuries was the main settlement on Portsea Island.

The parish church of Portsea is dedicated to St Mary and is found in the centre of Portsea island. Southsea and other areas did not come into being until the 19th century as housing was needed to house the sudden growth in population.

The name Portsea is said by some to be a mixture of PORT from Portsmouth and  SEA from Southsea.