Santi Sebastiano e Valentino was a small confraternity church that used to be on the Piazza Paganica. This is in the rione Sant'Angelo.
The church first appears in the list of churches dependent on San Lorenzo in Damaso, published in 1186 by Pope Urban III. Back then, it was simply known as San Valentino. An early cognomen was de Balneo Miccine, and by the 15th century this had become in Piscina. The Catalogus Cencii Camerarii from the end of the 12th century has a version of the former name, but Mabillon noted a manuscript giving the odd name of de Roxomiximo.
The legend grew up that the church was on the site of the house of St Valentine, but there is no early evidence of this.
Later in the Middle Ages, the dedication to St Sebastian was added and the church became known also as San Sebastiano all'Olmo. Old guidebooks can be found using this name, also that of Santi Valentino e Sebastiano.
Pope Clement VIII (1592-1605) suppressed the parish, and gave the church to the Compagnia dei Mercanti in 1593. This was a guild of shopkeepers specializing in what would now be called fashion accessories, especially perfume and silk ribbons. The perfumers had a special celebration on the feast of Candlemas, which used to be known as the "Purification of the Virgin" (as if using perfume instead of washing could make you pure - perhaps there was a hint of Roman humour in this).
As a result, two more names in the sources are San Sebastiano dei Mercanti or San Valentino dei Mercanti.
The church was rebuilt or extensively restored in the early 18th century, the architect being Francesco Felice Pozzoli.
It was demolished in 1870, as part of a development project which resulted in the large modern building now occupying the north of the site.
A strange confusion has emerged in online sources, which mistakenly places this church near the basilica of Santi Apostoli. This seems to be because of the various names given to the replacement building being the same as those of more famous palazzi. Palazzo Moroni and Palazzo Guglielmi are two names quoted.
The old Piazza Paganica was wider to the north, and shorter to the west, than the one there now. If you look into the piazza from the Via Paganica, you will see a white building at the far end of the left hand side. The piazza narrows and continues down the right hand side of this building. The church used to be here, in the present roadway, separated by a narrow alleyway from the block occupied by the white building.
The façade containing the entrance used to be level with that of this building, and the right hand side wall of the church was just within the footprint of the large modern building on the right.
There was no proper façade, as the church was included in the larger domestic edifice that occupied the block. Instead, there was a doorway leading into a short corridor which emerged into the church which was aligned east to west. This was a simple rectangle, of four bays separated by pilasters supporting the ceiling vault.
A fresco over the entrance was executed by Felice Ottini, a pupil of Giacinto Brandi.
The main altar had an altarpiece of St Sebastian allegedly by the Cavalier d'Arpino. The right hand side altar showed St Joseph Being Warned by an Angel in a Dream by Ottini, and the left hand one showed St Valentine by a pupil of the Cavalier d'Arpino whose name was recorded as Giovanni Battista.
The ceiling vault had a fresco by Placido Romoli (1690-1750), who was from Messina in Sicily but worked in Rome.
(Online information on this church is very limited.)