Swithun (or Swithin; Old English: Swīþhūn; Latin: Swithunus; died 2 July 863) was an Anglo-Saxon bishop of Winchester and subsequently patron saint of Winchester Cathedral. His historical importance as bishop is overshadowed by his reputation for posthumous miracle-working. According to tradition, if it rains on Saint Swithun's bridge (Winchester) on his feast day (15 July) it will continue for forty days.

Swithun was Bishop of Winchester from his consecration on 30 October 852 until his death on 2 July 863. However, he is scarcely mentioned in any document of his own time. His death is entered in the Canterbury manuscript of the Anglo-Saxon Chronicle (MS F) under the year 861. He is recorded as a witness to nine charters, the earliest of which (S 308) is dated 854.

More than a hundred years later, when Dunstan and Æthelwold of Winchester were inaugurating their church reform, Swithun was adopted as patron of the restored church at Winchester, formerly dedicated to St Peter and St Paul. His body was transferred from its almost forgotten grave to Æthelwold's new basilica on 15 July 971; according to contemporary writers, numerous miracles preceded and followed the move.

The revival of Swithun's fame gave rise to a mass of legendary literature. The so-called Vita S. Swithuni of Lantfred and Wulfstan, written about 1000, hardly contains any biographical fact; all that has in later years passed for authentic detail of Swithun's life is extracted from a late eleventh-century hagiography ascribed to Goscelin of St Bertin's, a monk who came over to England with Hermann, bishop of Salisbury from 1058 to 1078. According to this writer Saint Swithun was born in the reign of Egbert of Wessex, and was ordained priest by Helmstan, bishop of Winchester (838-c. 852). His fame reached the king's ears, and he appointed him tutor of his son, Æthelwulf (alias Adulphus), and considered him one of his chief friends. However, Michael Lapidge describes the work as "pure fiction" and shows that the attribution to Goscelin is false.

Under Æthelwulf, Swithun was appointed bishop of Winchester, to which see he was consecrated by Archbishop Ceolnoth. In his new office he was known for his piety and his zeal in building new churches or restoring old ones. At his request Æthelwulf gave the tenth of his royal lands to the Church. Swithun made his diocesan journeys on foot; when he gave a banquet he invited the poor and not the rich. William of Malmesbury adds that, if Bishop Ealhstan of Sherborne was Æthelwulf's minister for temporal matters, Swithun was the minister for spiritual matters.

Swithun's best-known miracle was his restoration on a bridge of a basket of eggs that workmen had maliciously broken. Of stories connected with Swithun the two most famous are those of the Winchester egg-woman and Queen Emma's ordeal. The former is to be found in the hagiography attributed to Goscelin, the latter in Thomas Rudborne's Historia major (15th century), a work which is also responsible for the story that Swithun accompanied Alfred on his visit to Rome in the 850s. He died on 2 July 862. On his deathbed Swithun begged that he should be buried outside the north wall of his cathedral where passers-by should pass over his grave and raindrops from the eaves drop upon it.

Swithun's feast day in England is on 15 July and in Norway (and formerly in medieval Wales) on 2 July. He is also listed on 2 July in the Roman Martyrology. He was moved from his grave to an indoor shrine in the Old Minster at Winchester in 971. His body was probably later split between a number of smaller shrines. His head was certainly detached and, in the Middle Ages, taken to Canterbury Cathedral. Peterborough Abbey had an arm. His main shrine was transferred into the new Norman cathedral at Winchester in 1093. He was installed on a 'feretory platform' above and behind the high altar. The retrochoir was built in the early 13th century to accommodate the huge numbers of pilgrims wishing to visit his shrine and enter the 'holy hole' beneath him. His empty tomb in the ruins of the Old Minster was also popular with visitors. The shrine was only moved into the retrochoir itself in 1476. It was demolished in 1538 during the English Reformation. A modern representation of it now stands on the site.

The shrine of Swithun at Winchester was supposedly a site of numerous miracles in the Middle Ages. Æthelwold of Winchester ordered that all monks were to stop whatever they were doing and head to the church to praise God every time that a miracle happened. A story exists that the monks at some point got so fed up with this, because they sometimes had to wake up and go to the church three or four times each night, that they decided to stop going. St Swithun then appeared in a dream to someone (possibly two people) and warned them that if they stopped going to the church, then miracles would cease. This person (or persons) then warned the monks about the dream they had, and the monks then caved in and decided to go to the church each time a miracle happened again.