Southampton is a port city and unitary authority situated in Hampshire, England, within the United Kingdom. It lies at the northernmost point of Southampton Water, positioned at the confluence of the River Test and Itchen, with the River Hamble joining to the south. The city is located approximately 80 miles southwest of London, 20 miles west of Portsmouth, and 20 miles southeast of Salisbury. It forms part of the larger South Hampshire conurbation, which includes Portsmouth and several boroughs such as Havant, Eastleigh, Fareham, and Gosport.

The area has been inhabited since the Stone Age, with a Roman fortress settlement called Clausentum established after AD 43. Anglo-Saxon settlers later formed Hamwic, which evolved into Hampton before Viking raids led to the establishment of medieval Southampton in the 10th century. The town was granted city status in 1964, becoming the City of Southampton. Its history includes significant events such as the departure of the Mayflower and the RMS Titanic, as well as heavy bombing during the Second World War known as the Southampton Blitz.

The city features extensive medieval fortifications, including walls completed in the 15th century, with roughly half surviving along with thirteen original towers and six gates. Notable sites include God's House Tower, built in 1417 as England's first purpose-built artillery fortification, and King John's House. The urban landscape reflects its modern feel due to wartime destruction, yet it retains historic elements like the Bargate. The city is also home to the ruins of the Franciscan friary and remnants of the motte and bailey castle.

Southampton has a strong association with maritime history, serving as a major embarkation point for D-Day and the Battle of Agincourt. It was one of Europe's main ports for ocean liners and remains a hub for cruise ships, including the Cunard Line's transatlantic service to New York. The city is also known for the Spitfire, built locally, and its connection to Jane Austen, who lived there for several years. These historical ties define its cultural identity alongside its role as a significant retail destination in southern England.

The population was recorded at 271,173 as of 2016, though the 2021 census counted 248,920 residents. The economy is supported by notable employers such as the University of Southampton, Ordnance Survey, BBC South, and Associated British Ports. Transport access includes Southampton Airport in Eastleigh and extensive national rail connections to London, Birmingham, and Manchester. The city serves as a key logistical node with regular ferry services and proximity to major international airports like Heathrow and Gatwick.