The Teatro dell'Opera di Roma is an opera house in Rome, Italy. Originally opened in November 1880 as the 2,212 seat Costanzi Theatre, it has undergone several changes of name as well modifications and improvements. The present house seats 1,600.

The Teatro dell'Opera was originally known as the Teatro Costanzi after the contractor who built it, Domenico Costanzi. It was financed by Costanzi, who commissioned the Milanese architect Achille Sfondrini, a specialist in the building and renovation of theatres. The opera house was built in eighteen months, on the site where the house of Heliogabalus stood in ancient times, and was inaugurated on 27 November 1880 with a performance of Semiramide by Gioachino Rossini.

With a seating capacity of 2,212, the house had three tiers of boxes, an amphitheatre and two separate galleries, surmounted by a dome adorned with splendid frescoes by Annibale Brugnoli.

Costanzi was obliged to manage the theater himself. Under his direction, and despite financial problems, the opera house held many world premieres of operas, including Cavalleria Rusticana by Pietro Mascagni. For a brief period the theatre was managed by Costanzi's son, Enrico, who gained renown by organizing another great premiere, that of Tosca by Giacomo Puccini on 14 January 1900.

In 1907 the Teatro Costanzi was purchased by the impresario Walter Mocchi on behalf of the Società Teatrale Internazionale e Nazionale (STIN). In 1912 Mocchi's wife, Emma Carelli, became the managing director of the new Impresa Costanzi, as the theatre was later known, following various changes in the company structure. During the fourteen years of her tenure, major works which had not been performed before in Rome (or even in Italy) were staged. These included La fanciulla del West, Turandot and Il trittico by Giacomo Puccini; Parsifal by Richard Wagner; Francesca da Rimini by Riccardo Zandonai; Boris Godunov by Modest Mussorgsky; Samson et Dalila by Camille Saint-Saëns and many others. Diaghilev's Ballets Russes also performed.

In November 1926 the Costanzi was bought by the Rome City Council and its name changed to Teatro Reale dell'Opera. A partial rebuilding ensued, led by architect Marcello Piacentini and lasting fifteen months. The house re-opened on 27 February 1928 with the opera Nerone by Arrigo Boito.

Chief among several major changes was the relocated entrance, from the street formerly known as Via del Teatro (where the garden of the Hotel Quirinale is now) to the opposite side, where Piazza Beniamino Gigli exists today. In addition, the amphitheatre inside the theatre was replaced by a fourth tier of boxes (now the third tier) and the balcony. The interior was embellished by new stuccowork, decorations, and furnishings, including a magnificent chandelier measuring six meters in diameter and composed of 27,000 crystal drops. Following the end of monarchy, the name was simplified to Teatro dell'Opera and, in 1958, the building was again remodeled and modernized. Rome City Council again commissioned architect Marcello Piacentini, who radically altered the building's style, notably with regard to the facade, entrance and foyer, each of these taking the form we know today.

The seating capacity is about 1,600. The house was retrofitted with air-conditioning subsequent to a restoration, which provided improvements to the interior. The stucco work was completely restored, the great proscenium arch strengthened, and a parquet floor of solid oak blocks laid to replace the previous one.

The name "Teatro Costanzi" remains officially in use, to refer to the main auditorium.