town, Lazio (Latium) regione, central Italy. It lies in the Cosa River valley, at 1,647 feet (502 m) above sea level, just north of Frosinone city. Said to have been founded in 1830 BC as Alatrium (mentioned by the Greek geographer Strabo), it belonged to the confederation of the Hernici, an ancient people of Italy, and later passed under the dominion of Rome (306 BC).

Archaeologically the town is of significance for its great belt of cyclopean walls (6th century BC) that enclose the superb trapezoid Pelasgian (pre-Hellenic) acropolis, the walls of which are almost intact. The outer circle of walls, about 2.5 miles (4 km) long, supplemented at intervals by fine medieval towers, is penetrated by three massive gates. Other important buildings are the Casagrandi Palace (now the civic museum), the bishop's palace, the episcopal seminary, and the cathedral. Although it is a resort with rail connections to Rome and Naples, Alatri is also an agricultural and manufacturing centre. Pop. (2006 est.) mun., 28,078.