San Sebastianello was the 19th century chapel of a small Dominican convent just off the north-east corner of Piazza di Spagna, at Via di San Sebastianello 10. This is in the rione Campo Marzio.
The dedication was to St Sebastian.
This chapel can be confused with San Sebastiano al Palatino, which has also been nicknamed San Sebastianello in the past.
Before the 19th century, the street known as Via di San Sebastianello used to lead to the side steps leading up to Santissima Trinità dei Monti, with a driveway continuing to a farmhouse in some vineyards below the slope of the Pincian Hill. This driveway became the northern section of the street in the early 19th century, when the vineyards were built upon.
According to the Roma Segreta website, there used to be a small chapel dedicated to the saint where the street now turns the corner. This was allegedly destroyed in 1733 by a fall of masonry from the retaining wall of the Piazza della Trinità dei Monti above, and replaced by the arched aedicule now there. This used to contain a portrait of the saint, and the empty frame of this is still extant.
The usual sources on Roman church history do not mention any chapel, and the info.roma web-page for the aedicule dates it to 1570. Hence it was probably just a wayside shrine. San Sebastianello means "Little St Sebastian".
In 1835, the Dominicans established a convent at Via di San Sebastianello 10, in a new building. After 1870 it became their Generalate in Rome, or their headquarters, and the chapel of the convent was open to the public. A suggestion has been raised that the main altar was privileged for the sake of pilgrims, so that they could gain the same indulgence at it as if they had visited San Sebastiano fuori le Mura. This needs documentary cofirmation.
There was a thorough re-fitting of the chapel in 1885.
In the 1930's the Dominicans made a deal with the Fascist government, whereby they gave up the convent in exchange for a tenancy at Santa Sabina which they used to own. The convent became a police headquarters, and the chapel was deconsecrated.
Number 10 is round the corner, below the road ramp and on the way to the church of Resurrezione di Nostro Signore Gesù Cristo dei Polacchi. There is nothing to see.
The chapel had no independent architectural identity. There were three altars, the main one being dedicated to the saint. The left hand one was dedicated to Our Lady of the Rosary, and the right hand one to St Thomas Aquinas. The latter had an altarpiece by one Fra Luigi, a Spanish Dominican.