St. Antimo Abbey (Abbazia di Sant'Antimo) is a former Benedictine monastery, now inhabited by Canons Regular, situated in the province of Siena, near Montalcino, Tuscany, central Italy. A tributary of the river Orcia, the Starcia, runs near the abbey.
Its name refers to Saint Anthimus of Rome, whose relics were moved here during the late 8th century. A Bollandist historian of the 17th century speculated that Pope Hadrian I gave the relics of Saints Sebastian and Anthimus to Charlemagne, who then donated them to the abbey when it was founded. But this theory has not yet been verified.

The origins of the abbey date to a small oratory built here, one the location of a former Roman villa, in 352 at the death of Anthimus. In 715 it was cared by the diocese of Chiusi.
In 770 the Lombards commissioned the construction of a Benedictine monastery, which had also to act as a hotel for the pilgrims directed to Rome. In 781, on his trip back from Rome, Charlemagne, gave his imprint to the construction, though the version according to which he was the founder of the abbey is most likely a legend. In 814 a document by Louis the Pious made it an imperial abbey. Later the abbot received the title of Palatine Count. At his apex, the abbey possessed 96 castles, terrains and other lands, as well as 85 monasteries, churches and hospitals. Their most important possession was the castle of Montalcino, which was the abbot's residence.


In 1118, after the donation of Bernardo degli Ardengheschi's heritage, the construction of the new church was started, inspired to the new abbey of Cluny and the church of Vignory.
In the mid-12th century, halted in its expansion northwards by Florence, Siena moved its attention to Montalcino. In 1145 the monks were forced to cease the castle of Radicofani the Sienese. In 1189 Pope Clement III placed the pieve of Montalcino under the bishop of Siena. Filippo Malavolti, podestà of Siena, attacked and partly destroyed Montalcino in 1200. An agreement of 1212 stated that the abbey had to hand over a quarter of its territories to Siena, including Montalcino.
In 1291 Pope Nicholas IV ordered the union of the abbey with the Guglielmites, a reformed branch of the Benictines, in order to give back strength to the abbey. However, after another period of decay in the 15th century, Pope Pius II annexed St. Anthimus to the new diocese of Montalcino-Pienza (1462), whose bishop was Pius' nephew. The abbey decayed to the point that in the 19th century it was used as stable. In 1870s the Italian state restored it.
In 1992 the abbey became again an active monastery with the arrival of a new religious community of Canons Regular of the Order of Premontre.