The placename is of uncertain origin. Some identify the town with anEtruscanVerentum, others trace the name toontano, Italian foralder, since alders cover the slopes of a nearby valley:Valle Ontano becomingValentano.
The town is named for the first time in a manuscript of 813 in theFarfa Register; starting in 844 a "Balentanu" appears in other documents of the abbey of San Salvatore onMt. Amiata. The land was definitely inhabited in prehistoric times, and important finds in theLake Mezzano and near Mt. Becco, Mt. Saliette, the Poggi del Mulino and Mt. Starnina seem to confirm the theories of historians, who identify the lake with the Lake of Statonia (Lacus Statoniensis) described bySeneca in hisNaturales Quaestiones and byPliny the Elder (ii.209,xiv.67 andxxxvi.168).
In the Renaissance period, the town fell under the dominion of theFarnese family: it is to them that Valentano owes its fortress (Rocca) and many of its churches.
In 1649, when the town ofCastro, capital of theDuchy of Castro, was destroyed, Valentano became the natural center of the Castrense region and the custodian of its archives.
In June 1944, an artillery shell exploded in the "Portonaccio" gate, killing seven civilians who had taken shelter in it. The gate itself is a witness to the tragedy, since one of its stones is missing, but in 2004 a plaque was placed in the Via Trento e Trieste to commemorate all local victims ofWorld War II.