Castello di Sidney Sonnino, Medieval castle in Montespertoli, Italy.

Castello di Sidney Sonnino is a medieval castle featuring a 13th-century tower that overlooks the surrounding valley, alongside a 17th-century palace and farm buildings. The complex displays multiple periods in its architecture and preserves both historical spaces and functional rooms for visitors to explore.

The castle, once called "di Montespertoli", was of medieval origin and belonged to the Ghibelline Alberti counts, lords of the area, until their line died out in 1393. It then passed to the Machiavelli, who already held estates in the territory, and from 1830 to the Sonnino family. Sidney Sonnino lived there and hosted figures such as Crispi, Giolitti, D'Annunzio, Umberto I and Vittorio Emanuele III, a period that saw major agricultural reforms and innovations.

The castle houses important documents related to Italy's involvement in World War I and international peace conferences. These archives give the place a role as a repository of national historical memory that visitors can explore.

The castle stands on a hill above the surrounding valley in Montespertoli. The site is reached by road from the town; the tower and palace are visible from the approach. The complex includes the medieval tower, the 17th-century palace and other buildings, so allow time to walk between them.

The castle's archive holds Sidney Sonnino's papers on the preliminaries and conduct of Italy's entry into the war in 1915, along with documents from the Paris congress of 1916 and the Versailles congress of 1918. Two paintings once kept here are now elsewhere: a Madonna and Child by Filippo Lippi in the local museum of sacred art, and a wooden crucifix by Taddeo Gaddi in the church of San Lorenzo at Montegufoni.