San Salvatore in Thermis was a small 10th century (?) church that used to be located on the south side of Via del Salvatore, which itself runs along the south side of San Luigi dei Francesi in the rione Sant'Eustachio.
The dedication was to Jesus Christ the Saviour, and it was nicknamed San Salvatorello because of its diminutive size.
Thermis referred to ancient Roman baths located here, the "Baths of Nero". This was the first imperial bathing complex in the city, commissioned by the emperor Nero (AD 37-68) and built by the government out of public funds. It was located between the Stadium of Domitian (the present Piazza Navona) and the Pantheon, and was restored by the emperor Alexander Severus in 227. From then on it was called the Thermae Alexandrinae.
The tiny church occupied the site of one of the dressing, resting and locker rooms to the west of the calidarium, and seemed originally to have been a re-use of this part of the ruinous ancient complex. One tradition alleged that it had been consecrated by Pope St Sylvester, and another that it had been consecrated by St Gregory the Great. The latter is possible although unprovable, and the church was probably not earlier than the 6th century because it is thought that the bath complex only became a ruin about then (the actual date of abandonment is unclear).
The first documentary record dates from 998. The site of the baths had come into the possession of the great Benedictine abbey of Farfa, and a document exists detailing an agreement between its abbot and the clergy in charge of Sant'Eustachio in Campo Marzio. Hülsen gives a quotation: Due ecclesiae Sanctae Mariae et Sancti Benedicti quae sunt aedificatae in thermis Alexandrinis, cum casis, criptis....et oratorio Salvatoris infra se. The church of St Benedict was later known as San Benedetto de Ferro, and the one dedicated to Our Lady as Santa Maria de Cella. Both of these were demolished for the French institutions attached to San Luigi.
The church was one of the dependents of San Lorenzo in Damaso in 1186, which indicates that it had a parochial function by then.
The abbey of Farfa kept possession of the monastic institution that it had here until 1478. Then the whole complex was sold to the French crown, which already had an adjacent small and cramped hospice for its pilgrims and expatriates which it wanted to expand. The result was the church of San Luigi and its attendant buildings, which took up most of the area. The original plan was that the part of the site south of Via del Salvatore, including San Salvatore, would become a large hospice for French pilgrims.
This part of the project stalled, however, and the site was sold to Giovanni de' Medici, the future Pope Leo X, in 1505. This was the origin of the famous Palazzo Madama. However, the French kept possession of the church which hence became a dependency of San Luigi and, as such, was apparently not much use to anyone.
The Palazzo was purchased by the Papal government for use as public offices in 1755, and hence became the property of the Italian government when Rome was conquered in 1870. The following year it became the seat of the Italian Senate (upper legislative chamber, as Italy has a bi-cameral system of legislature). The church was still not affected by all this, but security concerns were voiced about having a church with public access within the structure of the Palazzo. Fear of anarchist attacks prompted the closure of the church at the end of the 19th century (the French government seems not to have been worried).
The church was still standing in 1903 when Diego Angeli wrote a description of it, but was demolished soon after. Certain paintings were removed to San Luigi, and inscriptions to the courtyard of the Palazzo di San Luigi where they remain attached to a wall. Other fresco fragments were taken into the Palazzo Madama.
In 1907 Francesco Sabatini published a book on the church: La Chiesa di San Salvatore in Thermis, Il Salvatorello al Palazzo Madama. The Senate issued another book, San Salvatore in Thermis, Una Chiesa Scomparsa nell'Insula di Palazzo Madama, in 2012.
The church was not on the street, but parallel to it within the site of the palazzo. It was located at the north-west corner of the great court of the palazzo, and its location was due south of the westernmost three nave chapels of San Luigi which front onto the street.
The court has now been filled up with modern buildings, and the location of the church is inaccessible to the public.
This was a tiny church, just a rectangular box with a tiny niche on the left hand side containing a side altar. There was a minute narthex opening off the street through two doors, and matching two entrances leading into the right hand side of the church.
There was no proper façade. The right hand doorway had a Baroque doorframe with a curlicued device above it containing a 15th century bust of Christ the Saviour. The plainer left hand doorway had a large round-headed window above it which lighted the narthex.
The walls of the church were frescoed by Giovanni Odazzi (1663-1731); when the church was demolished, fragments were transferred to canvas and kept in the palazzo. The main altarpiece was an anonymous work of the 17th century, and this ended up in the sacristy of San Luigi. The holy water stoup was a re-used sarcophagus of a two-year-old child called Timoteo Cantabro. The side niche chapel contained the tomb of Reginaldo Campo Nivernese of 1460, together with 18th century stucco angels. Also noted was a 1523 tomb of Egidio de Hommedio, and a bas-relief featuring God the Father with Saints in the style of Giovanni Dalmata.