Portsea Island is a flat, low-lying island measuring 24.5 square kilometres in area, just off the southern coast of England. The island is located within the traditional and ceremonial county of Hampshire and contains the majority of the city of Portsmouth.
Portsea Island has the third-largest population of all the islands in the British Isles after the mainlands of Great Britain and Ireland; it also has the highest population density of any British Isle, and Portsmouth has the highest population density of any city in the UK outside of London.
To the east of Portsea Island lies Hayling Island, separated by Langstone Harbour. To the west is the peninsular mainland town of Gosport, separated by Portsmouth Harbour. To the south, it faces into the Spithead area of the wider Solent. A narrow tidal channel along the northern edge of Portsea Island, known as Portsbridge Creek, separates Portsea Island from the mainland.
Three main road bridges connect the island to the mainland road network. There is also a minor pedestrian and bicycle bridge at Hilsea Lines. In Portsmouth Harbour, a road bridge connects Portsea Island to Whale Island, a restricted Royal Navy shorebase establishment.