Santo Stefano al Casale dei Bagni dell'Acqua Santa is an 18th century farmstead chapel at Via dell'Acqua Santa 716/A in the Appio Pignatelli zone.
The dedication is to St Stephen the Protomartyr.
Acqua Santa ("Holy Water") is a rural locality just east of the Appian Way. The name comes from a natural chalybeate spring, the medicinal properties of which were allegedly discovered by a local farmer in the late 16th century. The appellation "Holy" is because Pope Paul V heard about this, and took the waters as part of a visit to the nearby San Sebastiano fuori le Mura.
He must have received some benefit, because a small country villa was constructed here for him and this became part of the patrimony of the Holy See. In 1779, Pope Pius VI ordered the construction of a much larger villa and this included the present chapel. This edifice became known as the Casale del Papa.
After the Napoleonic interregnum, the property passed to the Torlonia family and was renamed the Casale del Capitano all'Acquasanta. They oversaw a major remodelling, and the emergence of the spa as a popular destination for trips into the countryside by ordinary people. The water was also bottled commercially for sale.
At present (2018) the original public spa on the roadside is occupied by a restaurant called the Dea del Parco ("Goddess of the Park"). The villa and its chapel is behind this, invisible from the road.
The 18th century villa is a two-storey edifice of four conjoined wings around a tiny central courtyard garden. The chapel occupies the outside centre of the wing nearest to the road, and forms a short stub perpendicular to the major axis of the wing.
The façade facing the road is in a simple late Baroque style, with a single door-case having a lunette window above it and a triangular pediment. The window sits on a string course which runs round the sides of the chapel. The pediment does not occupy the full width, because the side corners are rounded.
This very obscure chapel seems always to have been private, and is not listed by the Diocese as a place of worship.