The Basilica of St Gregory the Great at Downside, commonly known as Downside Abbey, is a Catholic Benedictine monastery and the Senior House of the English Benedictine Congregation. One of its main apostolates is a school for children aged nine to eighteen. Its graduates are known as Old Gregorians.

Both monastery and school are located at between Westfield and Shepton Mallet in Somerset, south west England.

The community was founded at Douai, Flanders, then in the Spanish Netherlands, in 1605, under the patronage of St. Gregory the Great (who had sent St Augustine to England in 597). The founder was Saint John Roberts, who became the first prior and established the new community with other English monks who had entered various monasteries in the Spanish Benedictine Congregation, notably that at Valladolid. In 1611 Dom Philip de Caverel, abbot of St. Vaast's Abbey at Arras, built and endowed a monastery for the community.

The Priory of St. Gregory's, Douai, was therefore the first English Benedictine house to renew conventual life after the Reformation. For nearly 200 years the monastery trained monks for the English mission and six of these men were beatified by Pope Pius XI in 1929. Two of them, SS John Roberts and Ambrose Barlow, were among the Forty Martyrs of England and Wales canonized by Pope Paul VI in 1970.

Expelled from France during the French Revolution, after a period of imprisonment, in March 1795 the community were permitted to proceed to England. They settled for some 20 years as guests of Sir Edward Smythe at Acton Burnell, Shropshire, before finally settling at Mount Pleasant, Downside, in Somerset, in 1814.

The Monastery was completed in 1876 and the Abbey Church in 1925, being raised to the rank of a minor basilica in 1935 by Pius XI.

The building of Downside Abbey Church was begun in the 19th century, and ended with completion of the nave after World War I. The Abbey Church houses the relics of St Oliver Plunkett, an Irish martyr, who entrusted his body to the care of a Benedictine monk of the English Benedictine Congregation. The church is one of only three in the United Kingdom to be designated a minor basilica by the Roman Catholic Church, the others being St. Chad's Cathedral in Birmingham and Corpus Christi Priory, Manchester.

The church is built in the Gothic Revival style, and is designed to rival in size the medieval cathedrals of England that were lost to the Catholic Church through the Reformation. A magnificent ensemble of work is represented by a galaxy of architectural talent of the late nineteenth and early twentieth centuries. The earliest part is the richly decorated transepts by Archibald Matthias Dunn and Edward Joseph Hansom, dating from 1882. The choir is the masterpiece of Thomas Garner (who is buried there), dedicated in 1905. The soaring nave by Giles Gilbert Scott (c. 1923-25) remains unfinished, with its western wall in crude Lias stone standing bare and undecorated. The Lady Chapel is acknowledged as one of the most complete and successful schemes of Sir Ninian Comper, with a reredos and altar furnishings incorporating medieval fragments and a reliquary containing the skull of St Thomas de Cantilupe. The tower, completed in 1938, at 166 feet (55m), is the second highest in Somerset. The choir stalls are modelled on the famous stalls in Chester Cathedral.

Downside Abbey has been designated by English Heritage as a grade I listed building. Sir Nikolaus Pevsner described the Abbey as "the most splendid demonstration of the renaissance of Roman Catholicism in England. If ever there was an excuse for building in period forms in the twentieth century, it is here".