The Mariinsky Theatre is a historic theatre of opera and ballet in Saint Petersburg, Russia. Opened in 1860, it became the preeminent music theatre of late 19th-century Russia, where many of the stage masterpieces of Tchaikovsky, Mussorgsky, and Rimsky-Korsakov received their premieres. Through most of the Soviet era, it was known as the Kirov Theatre. Today, the Mariinsky Theatre is home to the Mariinsky Ballet, Mariinsky Opera and Mariinsky Orchestra. Since Yuri Temirkanov's retirement in 1988, the conductor Valery Gergiev has served as the theatre's general director.

The theatre is named after Empress Maria Alexandrovna, wife of Tsar Alexander II. There is a bust of the Empress in the main entrance foyer. The theatre's name has changed throughout its history, reflecting the political climate of the time.

The theatre building is commonly called the Mariinsky Theatre. The companies that operate within it have for brand recognition purposes retained the Kirov name, acquired during the Soviet era to commemorate the assassinated Leningrad Communist Party leader Sergey Kirov (1886–1934).

The Imperial drama, opera and ballet troupe in Saint Petersburg was established in 1783, at the behest of Catherine the Great, although an Italian ballet troupe had performed at the Russian court since the early 18th century. Originally, the ballet and opera performances were given in the wooden Karl Knipper Theatre on Tsaritsa Meadow, near the present-day Tripartite Bridge (also known as the Little Theatre or the Maly Theatre). The Hermitage Theatre, next door to the Winter Palace, was used to host performances for an elite audience of aristocratic guests invited by the Empress.

A permanent theatre building for the new company of opera and ballet artists was designed by Antonio Rinaldi and opened in 1783. Known as the Imperial Bolshoi Kamenny Theatre the structure was situated on Carousel Square, which was renamed Theatre Square in honour of the building. Both names – "Kamenny" (Russian word for "stone") and "Bolshoi" (Russian word for "big") – were coined to distinguish it from the wooden Little Theatre. In 1836, the Bolshoi Kamenny Theatre was renovated to a design by Albert Cavos (son of Catterino Cavos, an opera composer), and served as the principal theatre of the Imperial Ballet and opera.

On 29 January 1849, the Equestrian circus (Конный цирк) opened on Theatre Square. This was also the work of the architect Cavos. The building was designed to double as a theatre. It was a wooden structure in the then-fashionable neo-Byzantine style. Ten years later, when this circus burnt down, Albert Cavos rebuilt it as an opera and ballet house with the largest stage in the world. With a seating capacity of 1,625 and a U-shaped Italian-style auditorium, the theatre opened on 2 October 1860, with a performance of A Life for the Tsar. The new theatre was named Mariinsky after its imperial patroness, Empress Maria Alexandrovna.

Under Yuri Temirkanov, Principal Conductor from 1976 to 1988, the Opera Company continued to stage innovative productions of both modern and classic Russian operas. Although functioning separately from the Theatre’s Ballet Company, since 1988 both companies have been under the artistic leadership of Valery Gergiev as Artistic Director of the entire Theatre.

The Opera Company has entered a new era of artistic excellence and creativity. Since 1993, Gergiev’s impact on opera there has been enormous. Firstly, he reorganized the company’s operations and established links with many of the world's great opera houses, including the Royal Opera House, Covent Garden, the Metropolitan Opera, the Opéra Bastille, La Scala, La Fenice, the Israeli Opera, the Washington National Opera and the San Francisco Opera. Today, the Opera Company regularly tours to most of these cities.

Gergiev has also been innovative as far as Russian opera is concerned: in 1989, there was an all-Mussorgsky festival featuring the composer’s entire operatic output. Similarly, many of Prokofiev’s operas were presented from the late 1990s. Operas by non-Russian composers began to be performed in their original languages, which helped the Opera Company to incorporate world trends. The annual international "Stars of the White Nights Festival" in Saint Petersburg, started by Gergiev in 1993, has also put the Mariinsky on the world’s cultural map. That year, as a salute to the imperial origins of the Mariinsky, Verdi's La forza del destino, which received its premiere in Saint Petersburg in 1862, was produced with its original sets, costumes and scenery. Since then, it has become a characteristic of the "White Nights Festival" to present the premieres from the company’s upcoming season during this magical period, when the hours of darkness practically disappear as the summer solstice approaches.

Presently, the Company lists on its roster 22 sopranos (of whom Anna Netrebko may be the best known); 13 mezzo-sopranos (with Olga Borodina familiar to US and European audiences); 23 tenors; eight baritones; and 14 basses. With Gergiev in charge overall, there is a Head of Stage Administration, a Stage Director, Stage Managers and Assistants, along with 14 accompanists.