San Giovanni d’Asso is a frazione of the comune of Montalcino, province of Siena, Italy. It is located about 125 km southeast of Florence and about 40 km southeast of Siena in the area known as the Crete Senesi. It was a comune until 2017. The hamlet is overlooked by large Castle, now home to a large White truffle museum, and there is a festival celebrating the rare and fragrant tuber each year. The historical centre also houses the churches of San Giovanni Battista (pieve) and San Pietro in Villore, both of medieval origin.