|San Gregorio dei Muratori|
|English name:||St Gregory of the Wallers|
|Dedication:||Pope Gregory the Great|
|Address:||Via Leccosa 75|
San Gregorio dei Muratori is a tiny former confraternity church at Via Leccosa 75, hidden in a cul-de-sac between the Lungotevere Marzio and the Via Ripetta in the rione Campo Marzio.
It is a serious contender for the most unobstrusive church in the Centro Storico. Picture of the church on Wikimedia Commons. 
The dedication is to St Gregory the Great.
It was originally built in 1527 by the Confratarnita dei Muratori or "Confraternity of Wall-builders" (hence the name of the church -it does not automatically translate as "masons" because these only use stone not brick). The confraternity also came to include sculptors, stonecutters and plasterers and had a mainly expatriate Lombard membership.
They rebuilt it in 1599, from which date many of the surviving internal fittings come. The Nolli map of 1748 shows an edifice with a rectangular nave wider than it was long, with two pilasters on each side wall supporting the ceiling vault and a single entrance from the street. There was a square apse, on the other side of which was the river. To the left of the church was a separate oratory for the confraternity's private use, which was a rectangular building as long as the church.
The church was restored in 1748 by Cardinal Luigi Antonio Valdina Cremona, and it seems that the orientation was changed from perpendicular to the street to parallel with it. A watercolour of 1834 by Achille Pinelli shows two entrances in a single-storey façade of five vertical zones. The central zone, slightly brought forward, had a pair of Doric pilasters at its corners supporting an entablature with a triangular pediment. It displayed a large fresco of St Gregory Preaching, anonymous of the 18th century, as wide as the space below the pilasters and above a blank tablet in relief. The two entrances were in the main side zones, which had another pair of identical pilasters on their outer corners and supported a continuation of the entablature. The entrances themselves had Doric pilasters supporting raised segmental pediments with molded archivolts, and above them was a pair of square windows with stone frames. The two narrow outermost zones were set back again, were not as high as the main part of the façade and supported gigantic double volutes.
The old church was demolished, and its fittings stored, for the construction of an apartment block on the site in 1927. The ground floor of this was given over to be a new church, embellished with the original fittings, which was then given over to the care of the Arciconfraternita degli Amanti di Gesù e Maria al Calvario (Lovers of Jesus and Mary at Calvary) from 1934. They were responsible for the performance of the Stations of the Cross at the Colosseum, and until at least recently the large wooden cross that they used during the ceremonies was kept here.
After 1970, when the Roman rite of the Mass was revised, it was the only church in Rome at which the Mass in the Tridentine form was celebrated. At first this was unofficial, although condoned, but lately the church was administered by the Confraternity of Priests of St Peter (FSSP). However, when Santissima Trinità dei Pellegrini was erected as the central Roman parish of the Extraordinary form of the Mass, as it is now called, San Gregorio was closed and its future is now uncertain.
The church is unusual in that the exterior facade runs parallel to the nave. There is a pair of Doric pilasters flanking the original framed fresco of St Gregory, and either side of these pilasters is a doorway with a raised segmental pediment and a rectangular window above. The composition recalls that of the old church, but the façade is all in the same vertical plane and the pediment, outer pilasters and volutes are missing.
The nave is wider than it is long, a layout which derives from the original church and was because of the narrowness of the site between the street and the river before the Lungotevere was built. The interior has frescoes of the life of St Gregory, is decorated with stucco putti and gilding and has the original terracotta floors. The central elliptical panel of the ceiling, with a painting of the Allegory of the Trinity, is held up by four lifelike putti looking very precarious.
The main altar is in a rectangular apse with a barrel vault having rich stucco decoration, and was provided in the 1748 restoration.
Article in Italian (bad link)