| Castelfranco di Sotto | |
| Official Name: | Comune di Castelfranco di Sotto |
| Coordinates: | 43.7°N 55°W |
| Region: | Tuscany |
| Province: | Pisa (PI) |
| Frazioni: | Orentano, Villa Campanile, Galleno (partly), Staffoli (partly) |
| Mayor: | Umberto Marvogli |
| Area Total Km2: | 48.4 |
| Population Total: | 12179 |
| Population As Of: | 31 October 2008[1] |
| Elevation M: | 16 |
| Saint: | San Severo |
| Day: | November 18 |
| Postal Code: | 56022 |
| Area Code: | 0571 |
Castelfranco di Sotto is a comune (municipality) in the Province of Pisa in the Italian region Tuscany, located about 40km (30miles) west of Florence and about 30km (20miles) east of Pisa.
Castelfranco di Sotto borders the following municipalities: Altopascio, Bientina, Fucecchio, Montopoli in Val d'Arno, San Miniato, Santa Croce sull'Arno, Santa Maria a Monte.
Castelfranco is an ancient medieval village, whose name appeared for the first time in 1215. Tired of the battles fought in the area at the time between Florence, Lucca and Pisa, the population of nearby villages Vigesimo, Catiana, Paterno and Carpugnana decided to build here a defensive castle, named Castello di Franco, later changed to Castelfranco.
In 1966 Castelfranco was flooded by the river Arno.
Every year in Castelfranco the Palio dei Barchini is held.