Portsmouth is a port city in the ceremonial county of Hampshire on the south coast of England. Located mainly on Portsea Island, 70 miles south-west of London and 19 miles south-east of Southampton, it is the United Kingdom's only island city. It has a population of 205,400, and is the only city in the British Isles with a greater population density than London. The city forms part of the South Hampshire built-up area, which also covers Southampton and the towns of Havant, Waterlooville, Eastleigh, Fareham, and Gosport.

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 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 . Nearby Southsea is a seaside resort with a pier amusement park and medieval castle.

Portsmouth F.C. is the city's professional association football club and play their home games at Fratton Park. The city has several mainline railway stations that connect to London Waterloo amongst other lines in southern England. Portsmouth International Port is a commercial cruise ship and ferry port for international destinations. The port is the second busiest in the United Kingdom after Dover, handling around three million passengers a year. The city formerly had its own airport, Portsmouth Airport, until its closure in 1973. The University of Portsmouth enrols 23,000 students and is ranked among the world's best modern universities. Portsmouth is also the birthplace of author Charles Dickens and engineer Isambard Kingdom Brunel.