Southwark (/ˈsʌðərk/ ^ⓘ SUDH-ərk) is a district of Central London situated on the south bank of the River Thames, forming the north-western part of the wider modern London Borough of Southwark. The district, which is the oldest part of South London, developed due to its position at the southern end of the early versions of London Bridge, for centuries the only dry crossing on the river. Around 43 AD, engineers of the Roman Empire found the geographic features of the south bank here suitable for the placement and construction of the first bridge.

London's historic core, the City of London, lay north of the bridge and for centuries the area of Southwark just south of the bridge was partially governed by the City, while other areas of the district were more loosely governed. The section known as Liberty of the Clink became a place of entertainment. By the 12th century Southwark had been incorporated as an ancient borough, and this historic status is reflected in the alternative name of the area, as Borough. The ancient borough of Southwark's river frontage extended from the modern borough boundary, just to the west of the Oxo Tower, to St Saviour's Dock (originally the mouth of the River Neckinger) in the east. In the 16th century, parts of Southwark near London Bridge became a formal City ward, Bridge Without.

The urban area expanded over the years and Southwark was completely separated administratively from the now small City in 1900, although some remnants of City administration remain in the Guildable Manor. Like other parts of London, it now falls under the Mayor of London and London Assembly in addition to its Burough authority. Local points of interest include Southwark Cathedral, Borough Market, Shakespeare's Globe theatre, The Shard, Tower Bridge, Butler's Wharf and the Tate Modern museum.

The name Suthriganaweorc or Suthringa geweorche is recorded for the area in the 10th-century Anglo-Saxon document known as the Burghal Hidage and means "fort of the men of Surrey" or "the defensive work of the men of Surrey". Southwark is recorded in the 1086 Domesday Book as Sudweca. The name means "southern defensive work" and is formed from the Old English sūþ (south) and weorc (work). The southern location is in reference to the City of London to the north, Southwark being at the southern end of London Bridge. In Old English, Surrey means "southern district (or the men of the southern district)", so the change from "southern district work" to the latter "southern work" may be an evolution based on the elision of the single syllable ge element, meaning district.

Recent excavation has revealed pre-Roman activity including evidence of early ploughing, burial mounds and ritual activity.

The natural geography of Southwark (now much altered by human activity), was the principal determining factor for the location of London Bridge, and therefore London itself.

Until relatively recent times, the Thames in central London was much wider and shallower at high tide. The natural shoreline of the City Of London was a short distance further back than it is now, and the high tide shoreline on the Southwark side was much further back, except for the area around London Bridge.

Southwark was mostly made up of a series of often marshy tidal islands in the Thames, with some of the waterways between these island formed by branches of the River Neckinger, a tributary of the Thames. A narrow strip of higher firmer ground ran on a N-S alignment and, even at high tide, provided a much narrower stretch of water, enabling the Romans to bridge the river.