It was built as Benedictine Abbey, located in an important road junction, close to the other Abbey of Santa Maria Valdiponte (Montelabate). The first documentation of the Abbey dates back to 1156. The Abbey prospered until the end of the XII century, as the center of a vast agricultural area called the "Massa d 'Arna", belonging to the Pope. Later in 1237, due to its economic decline, Pope Gregory IX decided to entrust it to the Order militia and Temples in Tuscia, to make it an agricultural preceptory aimed at producing resources for the crusades. The agricultural preceptory was divided into two distinct domus: the first kept the original name of Giustino; the second, founded in 1256, was the Perugian Templar settlement of San Girolamo and San Bevignate.
Except for some period in which it was under the ancient Benedictine matrix, San Giustino remained a Templar preceptory until the temple order was suppressed in 1312. From 1316, it became a commendation of the Hospitaller Order of San Giovanni Gerosolimitano, later transformed into the Sovereign Military Order of Malta, which for seven centuries managed the site, today a farm.
Today only remains the ancient Abbey’s church with a basilical plan, restored in 1933 and renovated to the Romanesque style. The characteristic element of the church is the apse, although its decoration remains unfinished.
The interior has a single nave in the front area and a double in the Presbytery area. The ceiling is with wooden trusses.
The presbytery preserves a XV century fresco of Umbrian School which represents the Martyr San Giustino, with the millstone around the neck which, it is said, he was drowned with.
It is believed that the crypt, illuminated by loopholes, is the original Benedictine church, perhaps older than the year one thousand.Two squat columns with figured capitals (XI and XII century) support the cross vaults. At the back, three apses: in the centre one a XIV century fresco (1325-30) can be seen, depicting the crucifixion inspired by the Lorenzetti of the Basilica Assisiate, by an unknown author.