San Gemini is a comune of c. 5,000 inhabitants in the province of Terni in the Italian region Umbria, located about 60 km south of Perugia and about 10 km northwest of Terni.
San Gemini borders the municipalities of Montecastrilli, Narni and Terni.
The town is a well-preserved medieval burgh with two lines of walls, built over the remains of a small Roman center along the old Via Flaminia. It is especially known for its mineral waters.