Zagarolo is a town and comune in the Metropolitan City of Rome, in the region of Lazio of central Italy. It lies 34 kilometres (21 mi) southeast of Rome, and it borders the municipalities of Colonna, Gallicano nel Lazio, Monte Compatri, Palestrina, Rome, San Cesareo (former frazione of Zagarolo).

Zagarolo's town centre lies on a very narrow tuff hill, 2 kilometres (1.2 mi) long and about 100 metres (330 ft) wide, at an elevation of 310 metres (1,020 ft), surrounded by green valleys. The southern continuation of an important pilgrim route, the Via Francigena passes through the Zagarolo countryside.

Zagarolo may have origins in Gabii, an ancient city founded in the 5th century, and sited a few kilometres from Zagarolo. It was an ancient Roman town and there are remains of a Roman amphitheatre.

Zagarolo's town centre is of medieval origin and its current urban plan was developed in the 16th century. The town is the birthplace of composer Goffredo Petrassi.

Most of the main attractions of Zagarolo are in the historical town centre, concentrated around the narrow 1.5-kilometre (0.9 mi) long main street (Via Antonio Fabrini and then Corso Vittorio Emanuele).

The Palazzo Rospigliosi, in the city centre, occupies the site of an ancient medieval castle, which is first referenced to when Pope Paschal II destroyed Zagarolo in the early 12th century after the rebellion of the Colonna family, of which the palace was one of the strongholds. For many centuries it remained under the Colonna influence, and was besieged and destroyed several times due to the rivalry between the papacy and the Colonna family. In the 16th century, the Colonnas and the papacy started having peaceful relations, and Zagarolo became a duchy, hence the Palace became known as Palazzo Ducale.

The palace came to be used as a residence and two additional major wings were built towards the piazza, frescoes were added and a hanging garden was built. It was in this Palace that in 1591 a commission of eight cardinals met to revise the Bible for a printed edition, among them cardinal Marcantonio Colonna and Saint Robert Bellarmine. The building is characterized by frescoes painted by mannerist artists of the 16th century, attributed to Dutch painters, to Antonio Tempesta, and to the Zuccaris (Taddeo Zuccari and Federico Zuccari). Here in 1606 Caravaggio created masterpieces for the Colonna family, in exchange for refuge on his journey to Naples, between Zagarolo and Paliano he executed the Supper at Emmaus, Mary Magdalen in Ecstasy and perhaps Saint Francis in Prayer. The Palace also saw the of Carlo Maratta and of Ludovico Gimignani, the latter dying there in 1697.

In 1668, when the Rospigliosi acquired the duchy of Zagarolo, the Palace became their property, and was later in possession of the Pallavicini family, until it was sold to the comune in 1979. During World War II the palace was transformed into a German military hospital. Today the local library and Toy Museum are housed in the Palace, and it is used for conventions, exhibitions and other cultural events.