Castello di Montemassi, Medieval ruins in Roccastrada, Italy.

Castello di Montemassi is a medieval fortress whose stone structures rise from a hilltop in the Tuscan countryside. The remaining walls and tower fragments reveal a layout built for defense, with thick masonry and strategic angles that once protected the site.

Built shortly after the year 1000 as an Aldobrandeschi possession, the castle fell to Siena after a violent siege in the second half of the 13th century. In 1328 Guidoriccio da Fogliano retook it for Siena, an event depicted in Simone Martini's fresco in Siena's Palazzo Pubblico. After passing to the Salimbeni in 1374 and later the Verdelli of Montalcino, it was absorbed into the Republic of Siena in the early 15th century. In 1632 Grand Duke Ferdinando II de' Medici granted the complex to Gian Cristoforo II Malaspina; abandoned around 1840, it was restored between the late 20th century and the early 2000s.

The name Montemassi comes from the castle itself, and the residents' long struggle for a free comune is remembered locally. Since July 2023 the restored inner area has hosted cultural events organised by the Roccastrada municipality, with support from the University of Florence and the Colline Metallifere technological and archaeological park.

The inner area of the castle reopened on 13 July 2023 after redevelopment works completed on 4 July 2023, carried out with the University of Florence and the Parco nazionale tecnologico e archeologico delle Colline Metallifere, and is now used for cultural events organised by the municipality. Access is on foot through the village of Montemassi, a frazione of Roccastrada.

The 1328 siege led by Guidoriccio da Fogliano is depicted in Simone Martini's fresco in the Palazzo Pubblico of Siena, so the castle appears in one of the most famous images of medieval Siena. The fortress was finally abandoned around 1840 and restored between the late 20th century and the early 2000s.