The Abbey of Sant'Antimo, Italian: Abbazia di Sant'Antimo, is a former Benedictine monastery in the comune of Montalcino, Tuscany, central Italy. It is approximately 10 km from Montalcino about 9 km from the Via Francigena, the pilgrim route to Rome. After many years of disuse, the abbey was reoccupied in 1992 by a small community of Premonstratensian Canons Regular. Since January 2016, the occupants are a community of monks of the Olivetan Benedictine order.
A tributary of the river Orcia, the Starcia, runs near the abbey.
The name of the abbey may refer to Saint Anthimus of Rome, whose relics were supposedly moved here during the late 8th century.
The origins of the abbey are obscure. Archeological investigation of the site is incomplete, but has yielded artefacts from Late Classical times. The foundation of the original Benedictine monastery dates to the time of Charlemagne. The various accounts of Charlemagne founding the abbey are without direct historical foundation; they first appear in a document of the emperor Henry III from 1051.
The earliest document relating to the abbey is a land grant of Louis the Pious dated December 813, now in the Archivio di Stato of Siena. The abbot received full temporal powers in an imperial document of about 952. Following a bequest of Bernardo degli Ardengheschi, construction of the present church was begun before 1118, a date which is inscribed on the altar step and on a column to the left of it. Parts of the earlier structure remain visible in the crypt and in the so-called Cappella Carolingia, or Carolingian chapel.