Belisario (Belisarius) is a tragedia lirica (tragic opera) in three acts by Gaetano Donizetti. Salvadore Cammarano wrote the Italian libretto after Luigi Marchionni's adaptation of Eduard von Schenk's [de] play, Belisarius, first staged in Munich in 1820 and then (in Italian) in Naples in 1826. The plot is loosely based on the life of the famous general Belisarius of the 6th century Byzantine Empire.

It premiered to critical and popular success on 4 February 1836 at the Teatro La Fenice, Venice, and was given many additional performances that season, although Donizetti scholar William Ashbrook notes that there would have been more had the opera not been presented so late in the season.

However, in spite of its initial short-term success and critical reaction, as represented by a review in La Gazzetta privilegiata which stated that "A new masterwork has been added to Italian music...Belisario not only pleased and delighted, but also conquered, enflamed and ravished the full auditorium", in the long run, had "Donizetti poured music of the calibre of his Lucia di Lammermoor into the score of Belisario the shortcomings of its wayward plot and dramatic structure would matter less". By April 1836, even the composer himself recognized that the work stood below Lucia in accomplishment.

With the success of Lucia di Lammermoor in September 1835, Donizetti moved on to stagings of Maria Stuarda, the first under that name at La Scala in late 1835. He had signed the contract in July 1835 to present Belisario in Venice, for what would be the first visit to that city since 1819, but it was not until October that the subject was finally agreed upon.

There followed discussions with impresario Natale Fabbrici about employing a Venetian librettist, Pietro Beltrame. However, not only did the composer prefer to work with a librettist known to him and with whom he could work in close proximity, but he had already begun working with Cammarano who was revising – to the composer's satisfaction – an earlier version of Belisario which the librettist had submitted to the Teatro San Carlo management in 1832.

Of greater concern to Donizetti was the singers who were to be engaged. Primarily, he was concerned about the identity of the leading tenor: "Until I know for sure, I cannot compose duets, finales and trios", he writes in October. By the time Donizetti arrived in Venice on 6 January 1836, the score of Belisario was almost finished, and because of delays, he had time to hear several of the proposed singers in a performance of Rossini's L'assedio di Corinto given on 12 January, the day before rehearsals of Belisario were to begin.

In the case of Antonietta Vial (who was to sing the role of Irene and whom he described on first hearing her as "both a bastard soprano and a veiled contralto", he was able to make adjustments to suit her vocal limitations. By the time of the first performance, which was well received, Donizetti reported to his publisher the audience's reaction to most of the numbers, specifically that "in the duet for Vial and Salvatore, many shouts of bravi, but at the end (so they say) the situation is so moving that they were weeping".

In a review of a 2011 London performance, some of the strengths of Donizetti's score are outlined: