St Giles-in-the-Fields is the Anglican parish church of the St Giles district of London. The parish stands within the London Borough of Camden and forms part of the Diocese of London. The church, named for St Giles the Hermit, began as the chapel of a 12th-century monastery and leper hospital in the fields between Westminster and the City of London and now gives its name to the surrounding urban district of St Giles in the West End of London, situated between Seven Dials, Bloomsbury, Holborn and Soho. The present church is the third on the site since 1101 and was rebuilt most recently in 1731–1733 in Palladian style to designs by the architect Henry Flitcroft.

The first recorded church on the site was a chapel of the Parish of Holborn attached to a monastery and leper hospital founded by Matilda of Scotland, "Good Queen Maud", consort of Henry I between the years 1101 and 1109. The foundation would later become attached as a "cell," or subordinate house, to the larger Hospital of the Lazar Brothers at Burton Lazars, in Leicestershire. At the time of its founding it stood well outside the City of London and distant from the Royal Palace of Westminster, on the main road to Tyburn and Oxford. Between 1169 and 1189, on Michaelmas, Henry II granted the Hospital the lands, gifts and privileges that were to secure its future. For this he has been credited as a 'second founder'.

The chapel probably began to function as the church of a hamlet that grew up round the hospital. Although there is no record of any presentation to the living before the hospital was suppressed in 1539, the fact that the parish of St. Giles was in existence at least as early as 1222 means that the church was at least partially used for parochial purposes from that time.

The Precinct of the Hospital probably included the whole of the island site now bounded by High Street, Charing Cross Road and Shaftesbury Avenue. As well as the Hospital church which stood on the site of the present St Giles there would have been other buildings connected to the hospital including the Master's House (subsequently called the Mansion House) to the west of the church, and the 'Spittle Houses', dwellings attached to the Hospital on the eastern end of the present churchyard including the Angel Inn, which remains on the same site.

St Giles's position halfway between the ancient cities of Westminster and London is perhaps no coincidence. As George Walter Thornbury noted in London Old & New "it is remarkable that in almost every ancient town in England, the church of St. Giles stands either outside the walls, or, at all events, near its outlying parts, in allusion, doubtless, to the arrangements of the Israelites of old, who placed their lepers outside the camp."

During the 13th century a Papal Bull confirmed the hospital's privileges and granted it special protection under the See of Rome. The Papal Bull reveals that the lepers were trying to live as a religious community and that the Hospital precinct included gardens and 8 acres of land adjoining the Hospital to the north and south. The hospital was supported by the Crown and administered by the City of London for its first 200 years, being known as a Royal Peculiar.

In 1299, Edward I assigned it to Hospital of Burton Lazars in Leicestershire, a house of the order of St. Lazarus of Jerusalem, a chivalric order from the era of the Crusades. The 14th century was turbulent for the hospital, with frequent accusations of corruption and mismanagement from the City and Crown authorities and suggestions that members of the Order of Saint Lazarus (known as Lazar brothers) put the affairs of the monastery ahead of caring for the lepers. In 1348 The Citizens contended to the King that since the Master and brothers of Burton Lazars had taken over St. Giles's the friars had ousted the lepers and replaced them by brothers and sisters of the Order of St. Lazarus, who were not diseased and ought not to associate with those who were. The Hospital appears to have been governed by a Warden, who was subordinate to the Master of Burton Lazars. The King intervened on several occasions and appointed a new head of the hospital.

Eventually, in 1391, Richard II sold the hospital, chapel and lands to the Cistercian abbey of St Mary de Graces by the Tower of London. This was opposed by the Lazars and their new Master, Walter Lynton, who responded by leading a group of armed men to St Giles, recapturing it by force and by the City of London, which withheld rent money in protest. During this occupation the Order of St Lazarus opposed with armed force a visitation by the Archbishop of Canterbury and many important documents and records were lost or destroyed. The dispute was finally settled peacefully in court with the King claiming he had been misled about the ownership of St Giles and recognising Lynton as legal 'Master of St Giles Hospital' and the Hospital of Burton Lazarus with the Cistercian sale being formally revoked in 1402 and the property returned to the Lazar Brothers.