Glastonbury Abbey was a monastery in Glastonbury, Somerset, England. Its ruins, a grade I listed building and scheduled monument, are open as a visitor attraction.

The abbey was founded in the 8th century and enlarged in the 10th. Destroyed by a fire in 1184, it was subsequently rebuilt, and by the 14th century it was one of the richest and most powerful monasteries in England. The abbey controlled large tracts of the surrounding land and was instrumental in major drainage projects on the Somerset Levels. The abbey was suppressed during the Dissolution of the Monasteries under King Henry VIII of England. The last abbot, Richard Whiting (Whyting), was hanged, drawn and quartered as a traitor on Glastonbury Tor in 1539.

From at least the 12th century, the Glastonbury area has been associated with the legend of King Arthur, a connection promoted by medieval monks who asserted that Glastonbury was Avalon. Christian legends have claimed that the abbey was founded by Joseph of Arimathea in the 1st century.

Suggestions that Glastonbury may have been a site of religious importance in Celtic or pre-Celtic times are considered dubious by the historian Ronald Hutton, but archaeological investigations by the University of Reading have demonstrated Roman and Saxon occupation of the site. In 1955 Ralegh Radford's excavations uncovered Romano-British pottery at the west end of the cloister. The abbey was founded by Britons and dates at least to the early-7th century. Medieval occupation of the site is evidenced by pieces of ceramic wine jars that were imported from the Mediterranean. A medieval legend claimed that the abbey was founded by Joseph of Arimathea in the 1st century. This legend is intimately tied to Robert de Boron's version of the Holy Grail story and Glastonbury's connection with King Arthur from the early-12th century. William of Malmesbury reports the terms of a grant of land made by King Gwrgan of Damnonia to the "old church" at Glastonbury in AD 601 in the time of Abbot Worgret.

Glastonbury fell into Saxon hands after the Battle of Peonnum in 658. Saxons under Cenwalh of Wessex conquered Somerset as far west as the River Parrett, perhaps with the intention of gaining control of the abbey. Cenwalh allowed the British abbot, Bregored, to remain in power, a move perhaps intended as a show of good faith to the defeated Britons. After Bregored's death in 669, he was replaced by an Anglo-Saxon, Berhtwald, but British monks remained for many years.

King Ine of Wessex enriched the endowment of the community of monks established at Glastonbury and reputed to have directed that a stone church be built in 712, the foundations of which form the west end of the nave. A glassworks was established at the site during the 7th century. Glastonbury was affected by unrest due to the Danish incursions into the area in the 9th century and it may itself have been attacked during the wars (though this is not certain). The contemporary reformed soldier Saint Neot was sacristan at Glastonbury before he founded his own establishment in Somerset. The abbey church was enlarged in the 10th century by the abbot of Glastonbury, Dunstan, the central figure in the 10th-century revival of English monastic life, who instituted the Benedictine Rule at Glastonbury. He also built the cloisters. Dunstan became Archbishop of Canterbury in 960. In 946, King Edmund was interred at Glastonbury. In 1016 King Edmund Ironside was also buried there. King Cnut's charter of 1032 was "written and promulgated in the wooden church at Glastonbury, in the king's presence".

The medieval Glastonbury Canal was built about the middle of the 10th century to link the abbey with the River Brue, a distance of about 1.75 kilometres (1,900 yd). Its purpose is believed to have been to transport stone to build the abbey, but later it was used to transport produce, including grain, wine and fish, from the abbey's outlying properties. Much of the building stone came from the abbey's quarries at Doulting, accessed by way of the River Sheppey at Pilton. From the 11th century, the abbey was the centre of a large water-borne transport network as further canalisations and new channels were made, including the diversion of the Brue to access to the estate at Meare and an easier route to the Bristol Channel. In the 13th century, the abbey's head boatman transported the abbot in an eight-oared boat on visits to the abbey's nearby manors. During the Middle Ages, bone fragments of Saint Caesarius of Terracina were translated to Glastonbury Abbey.

At the Norman Conquest in 1066, the wealth of Glastonbury made it a prime prize. William the Conqueror made Turstinus a Norman abbot there in 1086. Turstinus added to the church, unusually building to the east of the older Saxon church and away from the ancient cemetery, thus shifting the sanctified site. This was later changed by Herlewin, the next abbot, who built a larger church. Not all the new Normans were suitable heads of religious communities. In 1086, when the Domesday Book was commissioned, Glastonbury Abbey was the richest monastery in the country. About 1125, the abbot Henry of Blois commissioned a history of Glastonbury from the historian William of Malmesbury, who was a guest of the monks. His work "On the Antiquity of the Glastonese Church" was compiled sometime between 1129 and 1139 as part of a campaign to establish the abbey's primacy against Westminster. It is the source for much of our knowledge of the abbey's early history but is far below William's generally excellent standards: his acceptance of the monks' forged charters and unsubstantiated early legends is apparent and even his list of the community's abbots cannot be reconciled with 10th-century originals subsequently discovered. These problems and the discrepancies between "On the Antiquity" and his own later histories has led many scholars to assume that William's original text was more careful and its accounts of "Phagan" and "Deruvian", along with various passages about Arthur, were later additions meant to bolster the monks' case.