Upper Palaeolithic deposits, including bones of Homo sapiens, have been found in local caves, and artefacts dating from the Bronze Age to the Middle Iron Age have been found at Mount Batten, showing that it was one of few principal trading ports of pre-Roman Britannia dominating continental trade with Armorica. An unidentified settlement named TAMARI OSTIA (mouth/estuaries of the Tamar) is listed in Ptolemy's Geographia and is presumed to be located in the area of the modern city. An ancient promontory fort was located at Rame Head at the mouth of Plymouth Sound with ancient hillforts located at Lyneham Warren to the east, Boringdon Camp and Maristow Camp to the north.

The settlement of Plympton, further up the River Plym than the current Plymouth, was also an early trading port. As the river silted up in the early 11th century, mariners and merchants were forced to settle downriver, at the current day Barbican near the river mouth. At the time this village was called Sutton, meaning south town in Old English. The name Plym Mouth, meaning "mouth of the River Plym" was first mentioned in a Pipe Roll of 1211.
The name Plymouth first officially replaced Sutton in a charter of King Henry VI in 1440. See Plympton for the derivation of the name Plym.

During the Hundred Years' War a French attack (1340) burned a manor house and took some prisoners, but failed to get into the town. In 1403 the town was burned by Breton raiders. On 12 November 1439, the English Parliament made Plymouth the first town incorporated. In the late fifteenth century, Plymouth Castle, a "castle quadrate", was constructed close to the area now known as The Barbican; it included four round towers, one at each corner, as featured on the city coat of arms.

The castle served to protect Sutton Pool, which is where the fleet was based in Plymouth prior to the establishment of Plymouth Dockyard. In 1512, an Act of Parliament was passed to further fortify Plymouth. The work included defensive walls at the entrance to Sutton Pool (across which a chain was extended in times of danger). Defences on St Nicholas Island also date from this time, and a string of six artillery blockhouses were built, including one on Fishers Nose at the south-eastern corner of the Hoe. This location was further strengthened by the building of a fort (later known as Drake's Fort) in 1596; it was the site of the Citadel, established in the 1660s (see below).

During the 16th century, locally produced wool was the major export commodity. Plymouth was the home port for successful maritime traders, among them Sir John Hawkins, who led England's first foray into the Atlantic slave trade, as well as Sir Francis Drake, Mayor of Plymouth in 1581–2. Crews for the first English failed settlement attempt at Roanoke Colony in North America departed in 1587 under Sir Walter Raleigh's and Drake's leadership; returning bearing maize, tobacco and potatoes.
In 1588, according to legend, Drake insisted on completing his game of bowls on the Hoe before engaging the Spanish Armada. In 1620 the Pilgrims set sail for the New World from Plymouth, establishing Plymouth Colony – the second English colony in what is now the United States of America.

During the English Civil War Plymouth sided with the Parliamentarians and was besieged for almost four years by the Royalists. The last major attack by the Royalists was by Sir Richard Grenville leading thousands of soldiers towards Plymouth, but they were defeated by the Plymothians at Freedom Fields Park. The civil war ended as a Parliamentary win, but monarchy was restored by King Charles II in 1660, who imprisoned many of the Parliamentary heroes on Drake's Island. Construction of the Royal Citadel began in 1665, after the Restoration; it was armed with cannon facing both out to sea and into the town, rumoured to be a reminder to residents not to oppose the Crown. Mount Batten tower also dates from around this time.

Throughout the 17th century, Plymouth had gradually lost its pre-eminence as a trading port. By the mid-17th century, commodities manufactured elsewhere in England cost too much to transport to Plymouth, and the city had no means of processing sugar or tobacco imports, major products from the colonies. Local sailors turning to piracy such as Henry Every became infamous, celebrated in the London play The Successful Pyrate. It played a part in the Atlantic slave trade during the early 18th century, although it was relatively small.

In the nearby parish of Stoke Damerel the first dockyard, HMNB Devonport, opened in 1690 on the eastern bank of the River Tamar. Further docks were built here in 1727, 1762 and 1793. The settlement that developed here was called "Dock" or "Plymouth Dock" at the time, and a new town, separate from Plymouth, grew up. In 1712 there were 318 men employed and by 1733 the population had grown to 3,000 people.

Before the latter half of the 18th century, grain, timber and then coal were Plymouth's main imports. During this time the real source of wealth was from the neighbouring town of Plymouth Dock (renamed in 1824 to Devonport) and the major employer in the entire region was the dockyard. The Three Towns conurbation of Plymouth, Stonehouse and Devonport enjoyed some prosperity during the late 18th and early 19th century and were enriched by a series of neo-classical urban developments designed by London architect John Foulston. Foulston was important for both Devonport and Plymouth and was responsible for several grand public buildings, many now destroyed, including the Athenaeum, the Theatre Royal and Royal Hotel, and much of Union Street.

Local chemist William Cookworthy established his short-lived Plymouth Porcelain venture in 1768 to exploit the deposits of china clay that he had discovered in Cornwall. He was acquainted with engineer John Smeaton, the builder of the third Eddystone Lighthouse.

The 1-mile-long (2 km) Breakwater in Plymouth Sound was designed by John Rennie to protect the fleet moving in and out of Devonport; work started in 1812. Numerous technical difficulties and repeated storm damage meant that it was not completed until 1841, twenty years after Rennie's death. In the 1860s, a ring of Palmerston forts was constructed around the outskirts of Devonport, to protect the dockyard from attack from any direction.

Some of the most significant imports to Plymouth from the Americas and Europe during the latter half of the 19th century included maize, wheat, barley, sugar cane, guano, sodium nitrate and phosphate. Aside from the dockyard in the town of Devonport, industries in Plymouth such as the gasworks, the railways and tramways, and a number of small chemical works had begun to develop in the 19th century, continuing into the 20th century.

During the First World War, Plymouth was the port of entry for many troops from around the Empire. It was developed as a facility for the manufacture of munitions. Although major units of the Royal Navy moved to the safety of Scapa Flow, Devonport was an important base for escort vessels and repairs. Flying boats operated from Mount Batten.

During the Second World War, Devonport was the headquarters of Western Approaches Command until 1941, and Sunderland flying boats were operated by the Royal Australian Air Force. It was an important embarkation point for US troops for D-Day. The city was heavily bombed by the Luftwaffe, in a series of 59 raids known as the Plymouth Blitz. Although the dockyards were the principal targets, much of the city centre and over 3,700 houses were completely destroyed and more than 1,000 civilians lost their lives. This was largely due to Plymouth's status as a major port. Charles Church was hit by incendiary bombs and partially destroyed in 1941 during the Blitz, but has not been demolished. It has been designated as an official permanent monument to the bombing of Plymouth during World War II.

The redevelopment of the city was planned by Sir Patrick Abercrombie in his 1943 Plan for Plymouth whilst simultaneously working on the reconstruction plan for London. This initially included plans to expand the city into south east Cornwall, but these were abandoned after opposition from Cornwall County Council. Between 1951 and 1957 over 1000 homes were completed every year, mostly using innovative prefabricated systems of just three main types.

The Plan for Plymouth was, on the one hand, a template for the rapid reassembly of a destroyed city centre, but Abercrombie also took the opportunity to lay out a whole hierarchy of settlements across the city of communities, neighbourhoods and districts. Central to this was a revision of transport infrastructure that prioritised the position of the railway as a gateway to the city centre and provided in the long-term for a dual carriageway road by-pass that only finally came into being in the 1980s (forty years after being planned). The plan is the subject of Jill Craigie's documentary The Way We Live (1946).

By 1964 over 20,000 new homes had been built, transforming the dense overcrowded and unsanitary slums of the pre-war city into a low density, dispersed suburbia. Most of the city centre shops had been destroyed and those that remained were cleared to enable a zoned reconstruction according to his plan. In 1962 the modernist high rise of the Civic Centre was constructed, an architecturally significant example of mid-twentieth century civic slab-and-tower set piece. The Plymouth City Council allowed it to fall into disrepair but it was grade II listed in 2010 by English Heritage to prevent its demolition.

Post-war, Devonport Dockyard was kept busy refitting aircraft carriers such as the Ark Royal and, later, nuclear submarines. New light industrial factories were constructed in the newly zoned industrial sector, attracting rapid growth of the urban population. The army had substantially left the city by 1971, after barracks were pulled down in the 1960s, but the city remains home to 29 Commando Regiment Royal Artillery and also 42 Commando of the Royal Marines.

Plymouth lies between the River Plym to the east and the River Tamar to the west; both rivers flow into the natural harbour of Plymouth Sound. Since 1967, the unitary authority of Plymouth has included the, once independent, towns of Plympton and Plymstock which lie along the east of the River Plym. The River Tamar forms the county boundary between Devon and Cornwall and its estuary forms the Hamoaze on which is sited Devonport Dockyard.

The River Plym, which flows off Dartmoor to the north-east, forms a smaller estuary to the east of the city called Cattewater. Plymouth Sound is protected from the sea by the Plymouth Breakwater, in use since 1814. In the Sound is Drake's Island which is seen from Plymouth Hoe, a flat public area on top of limestone cliffs. The Unitary Authority of Plymouth is 79.83 square kilometres (30.82 sq mi). The topography rises from sea level to a height, at Roborough, of about 509 feet (155 m) above Ordnance Datum (AOD).

Geologically, Plymouth has a mixture of limestone, Devonian slate, granite and Middle Devonian limestone. Plymouth Sound, Shores and Cliffs is a Site of Special Scientific Interest, because of its geology. The bulk of the city is built upon Upper Devonian slates and shales and the headlands at the entrance to Plymouth Sound are formed of Lower Devonian slates, which can withstand the power of the sea.

A band of Middle Devonian limestone runs west to east from Cremyll to Plymstock including the Hoe. Local limestone may be seen in numerous buildings, walls and pavements throughout Plymouth. To the north and northeast of the city is the granite mass of Dartmoor; the granite was mined and exported via Plymouth. Rocks brought down the Tamar from Dartmoor include ores containing tin, copper, tungsten, lead and other minerals. There is evidence that the middle Devonian limestone belt at the south edge of Plymouth and in Plymstock was quarried at West Hoe, Cattedown and Radford.

On 27 April 1944 Sir Patrick Abercrombie's Plan for Plymouth to rebuild the bomb-damaged city was published; it called for demolition of the few remaining pre-War buildings in the city centre to make way for their replacement with wide, parallel, modern boulevards aligned east–west linked by a north–south avenue (Armada Way) linking the railway station with the vista of Plymouth Hoe.

A peripheral road system connecting the historic Barbican on the east and Union Street to the west determines the principal form of the city centre, even following pedestrianisation of the shopping centre in the late 1980s, and continues to inform the present 'Vision for Plymouth' developed by a team led by Barcelona-based architect David MacKay in 2003 which calls for revivification of the city centre with mixed-use and residential.

In suburban areas, post-War prefabs had already begun to appear by 1946, and over 1,000 permanent council houses were built each year from 1951 to 1957 according to the Modernist zoned low-density garden city model advocated by Abercrombie. By 1964 over 20,000 new homes had been built, more than 13,500 of them permanent council homes and 853 built by the Admiralty.

Plymouth is home to 28 parks with an average size of 45,638 square metres (491,240 sq ft). Its largest park is Central Park, with other sizeable green spaces including Victoria Park, Freedom Fields Park, Alexandra Park, Devonport Park and the Hoe. Central Park is the home of Plymouth Argyle Football Club and a number of other leisure facilities.

The Plymouth Plan 2019–2034 was published May 2019 and sets the direction for future development with a new spatial strategy which reinforces links with the wider region in west Devon and east Cornwall in its Joint Local Plan and identifies three development areas within the city: the City centre and waterfront; a 'northern corridor' including Derriford and the vacant airfield site at Roborough; and an 'eastern corridor' including major new settlements at Sherford and Langage.

Plymouth has a moderated temperate oceanic climate (Köppen Cfb) which is wetter and milder than the rest of England. This means a wide range of exotic plants, palm trees, and yuccas can be cultivated. The annual mean high temperature is approximately 14 °C (57 °F). Due to the moderating effect of the sea and the south-westerly location, the climate is among the mildest of British cities, and one of the warmest UK cities in winter. The coldest month of February is similarly moderate, having mild mean minimum temperatures between 3 and 4 °C (37 and 39 °F). Snow usually falls in small amounts but a noteworthy recent exception was the period of the European winter storms of 2009–10 which, in early January 2010, covered Plymouth in at least 1 inch (2.5 cm) of snow; more on higher ground. Another notable event was the 8 inches (20 cm) of snowfall between 17 and 19 December 2010 – though only 2 inches (5.1 cm) would lie at any one time due to melting. Over the 1961–1990 period, annual snowfall accumulation averaged less than 7 cm (3 in) per year.

South West England has a favoured location when the Azores High pressure area extends north-eastwards towards the UK, particularly in summer. Coastal areas have average annual sunshine totals over 1,600 hours.

Owing to its geographic location, rainfall tends to be associated with Atlantic depressions or with convection and is more frequent and heavier than in London and southeast England. The Atlantic depressions are more vigorous in autumn and winter and most of the rain which falls in those seasons in the south-west is from this source. Average annual rainfall is around 980 millimetres (39 in). November to March have the highest mean wind speeds, with June to August having the lightest winds. The predominant wind direction is from the south-west.

Typically, the warmest day of the year (1971–2000) will achieve a temperature of 28.6 °C (83 °F), although in July 2022 the temperature reached 33.9 °C (93.0 °F), the site record. On average, 4.25 days of the year will report a maximum temperature of 25.1 °C (77 °F) or above. During the winter half of the year, the coldest night will typically fall to −4.1 °C (25 °F) although in January 1979 the temperature fell to −8.8 °C (16 °F). Typically, 18.6 nights of the year will register an air frost.

People from Plymouth are known as Plymothians or less formally as Janners. Its meaning is described as a person from Devon, deriving from Cousin Jan (the Devon form of John), but more particularly in naval circles anyone from the Plymouth area.

The Elizabethan navigator, Sir Francis Drake was born in the nearby town of Tavistock and was the mayor of Plymouth. He was the first Englishman to circumnavigate the world and was known by the Spanish as El Draco meaning "The Dragon" after he raided many of their ships. He died of dysentery in 1596 off the coast of Portobelo, Panama. In 2002 a mission to recover his body and bring it to Plymouth was allowed by the Ministry of Defence. His cousin and contemporary John Hawkins was a Plymouth man. Painter Sir Joshua Reynolds, founder and first president of the Royal Academy was born and educated in nearby Plympton, now part of Plymouth. William Cookworthy born in Kingsbridge set up his successful porcelain business in the city and was a close friend of John Smeaton designer of the Eddystone Lighthouse. On 26 January 1786, Benjamin Robert Haydon, an English painter who specialised in grand historical pictures, was born here. The naturalist Dr William Elford Leach FRS, who did much to pave the way in Britain for Charles Darwin, was born at Hoe Gate in 1791.

Antarctic explorers Robert Falcon Scott who was born in Plymouth and Frank Bickerton both lived in the city. Artists include Beryl Cook whose paintings depict the culture of Plymouth and Robert Lenkiewicz, whose paintings investigated themes of vagrancy, sexual behaviour and suicide, lived in the city from the 1960s until his death in 2002. Illustrator and creator of children's series Mr Benn and King Rollo, David McKee, was born and brought up in South Devon and trained at Plymouth College of Art. Jazz musician John Surman, born in nearby Tavistock, has close connections to the area, evidenced by his 2012 album Saltash Bells. The avant garde prepared guitarist Keith Rowe was born in the city before establishing the jazz free improvisation band AMM in London in 1965 and MIMEO in 1997. The musician and film director Cosmo Jarvis has lived in several towns in South Devon and has filmed videos in and around Plymouth. In addition, actors Sir Donald Sinden and Judi Trott were born in Plymouth. George Passmore of Turner Prize winning duo Gilbert and George was also born in the city, as was Labour politician Michael Foot whose family reside at nearby Trematon Castle.

Notable athletes include swimmer Sharron Davies, diver Tom Daley, dancer Wayne Sleep, and footballer Trevor Francis. Other past residents include composer journalist and newspaper editor William Henry Wills, Ron Goodwin, and journalist Angela Rippon and comedian Dawn French. Canadian politician and legal scholar Chris Axworthy hails from Plymouth. America based actor Donald Moffat, whose roles include American Vice President Lyndon B. Johnson in the film The Right Stuff, and fictional President Bennett in Clear and Present Danger, was born in Plymouth. Canadian actor Mark Holden was also born in Plymouth.
Kevin Owen is an international TV news anchor who was born in Freedom Fields Hospital, while his father served as a Royal Navy Officer.
Cambridge spy Guy Burgess was born at 2 Albemarle Villas, Stoke whilst his father was a serving Royal Navy officer.