Alatri (Latin: Aletrium) is a town and comune of province of Frosinone in the Italian region of the Lazio, with c. 30,000 inhabitants. A part of the traditional region of Ciociaria, it is known for its megalithic acropolis.

The area of the modern city was settled as early as the 2nd millennium BC.

Aletrium was a town of the Hernici which, together with Veroli, Anagni and Ferentino, formed a defensive league against the Volsci and the Samnites around 550 BC. In 530 they allied with Tarquinius Superbus' Rome, confirming the Etruscan influence in the area attested also by archaeological findings. Alatri was defeated by Rome in 306 BC and forced to accept the citizenship. In Cicero's time it was a municipium (an ancient coat of arms can be seen with the write "S.P.Q.A."), and continued in this position throughout the imperial period.

After the fall of the Western Roman Empire, the city decayed, the only respected authority being represented by the Christian bishop. During the reign of Theodoric the Great (5th century AD) the patrician Liberius promoted the construction of a monastic community, one of the most ancient in the West, where in 528 St. Benedict sojourned briefly. In 543, during the Gothic Wars, Alatri was sacked and destroyed by Totila's troops.

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