Santa Maria dell'Arco in un Torrione was a tiny devotional chapel in one of the towers of the ancient Aurelian Walls on the present Viale Giotto. This is in the rione San Saba.
The Nolli Map of 1748 shows what is now the Viale Giotto as a country lane running from the Porta San Paolo along the inside line of the walls to Santa Balbina. Apart from this chapel, there was not a single building of any sort along the way and the land was given over to vineyards.
The chapel was in the sixth tower, counting west from the right-angled bend in the walls at the junction with Via Guerrieri and is the one just west of the junction with Via Andrea Palladio.
There seem to be no references online to its history, but the towers of the city walls were favoured as places for hermits to live in the early Middle Ages. This one was probably once a hermitage attached to the monastery of San Salvatore de Porta, which itself was a dependency of San Saba in the 10th century. Another example of a chapel in the walls deriving from a hermitage is Santa Margherita in Prigione.
The chapel probably did not survive the French occupation. The tower is now a ruin, covered in weeds and closed off with an iron railing gate.