Santa Maria in San Giovannino was a 12th century convent church, now demolished, on the Via del Moretto in the rione Colonna.
This lost church is not well known, and the information on it is muddled. For example, Armellini wrote that it was the same church as San Giovanni in Cliveo Plumbeo and that it was part of the monastery of San Silvestro, both of which assertions are incorrect. Also, in reality it remained as a functioning church until the end of the 18th century -and was not demolished in the 16th or 17th as also asserted.
Before the 13th century it was where the famous alleged relic of the head of St John the Baptist was venerated, and was hence known as San Giovanni in Capite.
The first reference to the presence of the head here is in 1140, although the tradition is that it was first brought to Rome by expatriate Greek monks in the 8th century. However scholarly suspicions have focused on the head of a martyr called St John the Priest, which was being venerated under the altar of his basilica in early mediaeval times. This was located at the Catacomba ad clivum Cucumeris on the Via Salaria Vetus, and the reasonable (although undocumented) suggestion is that this head was brought into the city in the 8th or 9th century and then mislabelled.
The relic was moved into the neighbouring church of San Silvestro in Capite in the 13th century, about the time when the Poor Clare nuns took possession of the attached monastery. This explains why the latter church was not dedicated to St John, despite having an alleged relic of one of the most important saints in Christendom.
In the late Middle Ages, the church was known as San Giovannino because of its small size.
By 1578 it was desecrated and derelict. In that year, it was reported that a miracle had been performed by a surviving image of Our Lady there, and the devotion that resulted led to a rebuilding with a new dedication to her. At first, the church was in the care of the Congregation of Christian Doctrine, which used it as a centre for their catechetical activities. However in 1600 the complex was passed on to the Discalced Mercedarians of Spain, and was placed under the protection of the King of Spain. The Discalced Mercedarians were a reform movement within the Mercedarian order, and the convent was their headquarters in Rome.
According to the Mercedarians now, the little monastery was closed down by the French occupiers very soon after their arrival in 1798. However, the church edifice survived and was being used as an annexe of the city's main post office (next door at the old convent of San Silvestro) in the late 19th century. It seems to have been demolished then, to be replaced by the present building.
The church was on the corner of Via della Mercede (which preserves the memory of the convent) and Via del Moretto. It was oriented west to east, with the main entrance on the latter street. The present building stands on the church's footprint at that street corner.
Despite its size the church had a basilical plan. There was a nave of four bays with aisles, and the columns of the arcades (three on each side) were reported to be ancient ones of granite. The north aisle had two external chapels, and at the end of each aisle was another chapel to each side of the presbyterium. There was a triumphal arch with large pillars, and this led into a presbyterium with apse.
The monastery was arranged around a cloister just north of the church, which had arcaded walks on the north, west and south sides and a fountain in the garth. This convent was only a quarter of the size of the Poor Clare establishment at San Silvestro.