The Theater of Marcellus (Latin: Theatrum Marcelli) is a theater from ancient Rome, still partially preserved.
Construction of the theater was begun by Julius Caesar, who expropriated a large swath of the area, demolishing the existing buildings. It was completed by Augustus, who dedicated the theater to his nephew Marcus Claudius Marcellus, son of his sister Octavia. Augustus had designated Marcellus as his heir, giving him his daughter Julia in marriage, but the young man died prematurely.

The semicircular auditorium, 130 meters in diameter, featured a travertine façade with three superimposed orders (Doric, Ionic, Corinthian) like those of the Colosseum, of which the first two remain today. The attic on the third floor, of which few traces remain, had a continuous wall. The theater reflects the layout of the classical Roman theater, in which the auditorium rests on masonry structures rather than on a natural slope, as in Greek theaters.

The theater's stage is completely lost, but a map of the ancient city of Rome engraved on marble slabs from the early 3rd century (Forma Urbis Severiana) shows the original structure decorated with columns and marble statues.
In 13 BC, the theater was officially inaugurated with sumptuous games and could accommodate over 15,000 spectators.

During the Middle Ages, it was gradually occupied by small buildings and was first transformed into a veritable fortress and later into a noble residence. The palace was raised above the still-standing portions of the Theatre of Marcellus in the early 16th century by the Savelli family, who had become its owners, and commissioned the architect Baldassarre Peruzzi to design the project.
In 1729, the palace was purchased by Ferdinando Bernualdo Filippo Orsini d'Aragona, Duke of Gravina, whose descendants still own part of the palace today.