| Piancastagnaio | |
| Official Name: | Comune di Piancastagnaio |
| Coordinates: | 42.85°N 52°W |
| Region: | Tuscany |
| Province: | Siena (SI) |
| Frazioni: | Saragiolo |
| Mayor: | Luigi Vagaggini |
| Area Total Km2: | 69.7 |
| Population Footnotes: | [1] |
| Population Total: | 4200 |
| Population As Of: | 28 February 2017 |
| Population Demonym: | Pianesi |
| Elevation M: | 772 |
| Postal Code: | 53025 |
| Area Code: | 0577 |
Piancastagnaio is a comune (municipality) in the Province of Siena in the Italian region Tuscany, located about 110km (70miles) southeast of Florence and about 60km (40miles) southeast of Siena. It is located on the Monte Amiata slopes.
The main attractions include the Pieve of Santa Maria Assunta, in Baroque style but existing from before 1188, the Palazzo Bourbon Del Monte and the Rocca Aldobrandesca ("Aldobrandeschi Castle").