Portsmouth is a port and island city with unitary authority status in the ceremonial county of Hampshire, southern England. It is the most densely populated city in the United Kingdom, with a population last recorded at 238,800. The city forms part of the South Hampshire built-up area, which also incorporates Southampton, Havant, Waterlooville, Eastleigh, Fareham, and Gosport. Located mainly on Portsea Island, Portsmouth is located 19 miles (31 km) south-east of Southampton and 70 miles (110 km) south-west of London.

The city's history can be traced to Roman times. A significant naval port for centuries, Portsmouth has the world's oldest dry dock and was England's first line of defence during the French invasion in 1545. Special Palmerston Forts were built in 1859 in anticipation of another invasion from continental Europe. By the early-19th century, Portsmouth was the most heavily fortified city in the world, and was considered "the world's greatest naval port" at the height of the British Empire throughout Pax Britannica. The world's first mass production line was set up in the city, making it the most industrialised site in the world. During the Second World War, the city was a pivotal embarkation point for the D-Day landings and was bombed extensively in the Portsmouth Blitz, which resulted in the deaths of 930 people. In 1982, the city housed the entirety of the attacking forces in the Falklands War. Her Majesty's Yacht Britannia left the city to oversee the transfer of Hong Kong in 1997, which marked for many the end of the empire. Portsmouth is one of the world's best known ports. HMNB Portsmouth is the largest dockyard for the Royal Navy and is home to two-thirds of the UK's surface fleet. The city is home to some famous ships, including HMS Warrior, the Tudor carrack Mary Rose and Horatio Nelson's flagship, HMS Victory (the world's oldest naval ship still in commission). The former HMS Vernon naval shore establishment has been redeveloped as a retail park known as Gunwharf Quays. Portsmouth is among the few British cities with two cathedrals: the Anglican Cathedral of St Thomas and the Roman Catholic Cathedral of St John the Evangelist. The waterfront and Portsmouth Harbour are dominated by the Spinnaker Tower, one of the United Kingdom's tallest structures at 560 feet (170 m). Nearby Southsea is a seaside resort with a pier amusement park and medieval castle.