Un ballo in maschera (A Masked Ball) is an opera in three acts by Giuseppe Verdi with text by Antonio Somma. However, Somma's libretto was itself based on the five act libretto which playwright Eugène Scribe had written for Daniel Auber's 1833 opera, Gustave III, ou Le bal masqué.

Scribe wrote about the assassination in 1792 of King Gustav III of Sweden who was killed as the result of a political conspiracy against him. He was shot while attending a masked ball and died 13 days later of his wounds.

It was to take over two years between the time of the commission from Naples and planned for a production there and its premiere performance at the Teatro Apollo in Rome on February 17, 1859. In order to become the Un ballo in maschera which we know today, Verdi's opera (and his libretto) was forced to undergo a significant series of transformations and title changes. Based on the Scribe libretto and begun as Gustavo III set in Sweden, it became Una vendetta in dominò set in Germany, and finally Un ballo, set not in Sweden but in Boston, Massachusetts during the colonial era. These changes were caused by a combination of censorship regulations in both Naples and Rome, as well as by the political situation in France in January 1858. It became one of the most frustrating experiences of Verdi's career.

From the mid-20th century, it has become more common for the setting to revert to its original 18th century Stockholm location. A re-creation of the original Gustavo III has been staged in Sweden.

This 1954 recording at Carnegie Hall stars Jan Peerce and Robert Merrill. Arturo Toscanini conducts the NBC Symphony and the Robert Shaw Chorale.