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Thornbury is a market town and civil parish in the South Gloucestershire unitary authority area of England, about 12 miles (19 km) north of Bristol. It had a population of 12,063 at the 2011 census, rising to 14,496 in the 2021 census. Thornbury is a Britain in Bloom award-winning town, with its own competition: Thornbury in Bloom. The earliest documentary evidence of a village at "Thornbyrig" dates from the end of the 9th century. The Domesday Book of 1086 noted a manor of "Turneberie" belonging to William the Conqueror's consort, Matilda of Flanders, with 104 residents.

There is evidence of human activity in the Thornbury area in the Neolithic and Bronze Ages, but evidence of the Roman presence is confined to the Thornbury hoard of 11,460 Roman coins dating from 260 to 348 CE, found in 2004 during the digging of a fishpond. The name Thornbury derives from the Old English Þornburh meaning 'thorn burh' (fortification). The earliest documentary evidence of a village at "Thornbyrig" dates from the end of the 9th century.

St Mary's Church, begun in the 12th century with later additions, is the oldest surviving building. The town charter was granted in 1252 by Richard de Clare, 6th Earl of Gloucester and lord of the manor of Thornbury. (The charter's 750th anniversary in 2002 was celebrated with a "750" flowerbed planted in Grovesend Road.) The town grew around the site of its cattle market. Thornbury lost its status as a borough in 19th-century local-government reforms, but in 1974 the parish council exercised its new right to designate itself a town council.

The ancient parish covered an area extending to the River Severn, including the detached area of Rangeworthy until 1866, when this became a separate civil parish. In 1894 the western part of Thornbury was detached to form the civil parish of Oldbury-on-Severn and the eastern part to create that of Falfield.

Thornbury Township, Pennsylvania, founded in 1687, was named after Thornbury, Gloucestershire, by George Pearce, whose wife Ann came from there. In 1765 Dr John Fewster of Thornbury presented a paper to the Medical Society of London entitled "Cow Pox and its Ability to Prevent Smallpox". Fewster influenced his friend and colleague Edward Jenner, pioneer of vaccination.

Thornbury was once the terminus of a Midland Railway (later LMS) branch line from Yate on the Bristol to Gloucester main line, with intermediate stations at Iron Acton and Tytherington. It lost its passenger services in June 1944 but continued as a goods route, also serving quarries at Tytherington. The site of Thornbury railway station and the line have been redeveloped into a supermarket, a housing estate, a bypass road and a long footpath. Further relics can be seen at Tytherington Quarry to the east of the town. There are plans to reopen the line to Yate via Tytherington and Iron Acton and possibly restore services to Gloucester and Bristol.

Thornbury held a market in the high street and the market hall. This closed in the late 1990s and was partly replaced by a smaller one in a car park near the United Reformed Church. The older site has been redeveloped as a community centre called "Turnberrie's"; the older community centre, at the Chantry in Castle Street, remains in active use. The old market hall is now a restaurant.

Thornbury's coat of arms combines the arms of four families important to its history: the Attwells – Howard, Clare and Stafford. John Attwells bequeathed £499.99 for the establishment of a free school that merged with the grammar school in 1879. Their arms were later adopted as the badge for the grammar school. The other three families held the manor at Thornbury over several centuries, with the Latin motto Decus Sabrinae Vallis (Jewel of the Severn Vale).

Thornbury has a radio station, Thornbury Radio, which broadcasts on 105.1 and 107.5 FM, and online from its studio at 12 The Plain.