Piansano is a (municipality) in the Province of Viterbo in the Italian region of Latium, located about northwest of Rome and about northwest of Viterbo. As of 31 December 2004, it had a population of 2,232 and an area of .
Piansano borders the following municipalities: Arlena di Castro, Capodimonte, Cellere, Tuscania, Valentano.