La sonnambula (Italian pronunciation: [la sonˈnambula]; The Sleepwalker) is an opera semiseria in two acts, with music in the bel canto tradition by Vincenzo Bellini set to an Italian libretto by Felice Romani, based on a scenario for a ballet-pantomime written by Eugène Scribe and choreographed by Jean-Pierre Aumer called La somnambule, ou L'arrivée d'un nouveau seigneur. The ballet had premiered in Paris in September 1827 at the height of a fashion for stage works incorporating somnambulism.

The role of Amina was originally written for the soprano sfogato Giuditta Pasta and the tenor Giovanni Battista Rubini, but during Bellini's lifetime another soprano sfogato, Maria Malibran, was a notable exponent of the role. The first performance took place at the Teatro Carcano in Milan on 6 March 1831.

The majority of twentieth-century recordings have been made with a soprano cast as Amina, usually with added top-notes and other changes according to tradition, although it was released in soprano sfogato voice (not be confused with the modern mezzo, nonexistent at the time) who sang soprano and contralto roles unmodified.

The phrase Ah! non credea mirarti / Sì presto estinto, o fiore ("I did not believe I would see you fade so soon, oh flower") from Amina's final aria is inscribed on Bellini's tomb in the Catania Cathedral in Sicily.

Returning to Milan after the I Capuleti e i Montecchi performances in March 1830, little occurred until the latter part of April when Bellini was able to negotiate a contract with both the Milan house for the autumn of 1831 and another for the 1832 Carnival season at La Fenice in Venice; these operas were to become Norma for La Scala and Beatrice di Tenda for Venice. Writing to his uncle in Sicily, the composer reported that "I shall earn almost twice as much as if I had composed [only for the Venetian impresario]".

However, there was also a contract for a second Milan house for the following winter season for as-yet an unnamed opera, but it had already been agreed that Giuditta Pasta, who had achieved success in Milan in 1829 and 1830 appearing in several major operas, would be the principal artist.

Then Bellini experienced the re-occurrence of an illness which had emerged in Venice due to pressure of work and the bad weather, and which consistently recurred after each opera. The gastro-enteric condition—which he described as "a tremendous inflammatory gastric bilious fever"— resulted in his being cared for by friends. It was not until the summer, when he went to stay near Lake Como, that the pressure to decide upon a subject for the following winter's opera became more urgent. That Pasta owned a house near Como and would be staying there over the summer was the reason that Felice Romani traveled to meet both her and Bellini.

By 15 July they had decided on a subject for early 1831, but it was uncertain as to whether Pasta was interested in singing a trousers role, that of the protagonist, Ernani, in an adaptation of Victor Hugo's Hernani, later set to music by Giuseppe Verdi in 1844. With both men having various other commitments, by the end of November 1830 nothing had been achieved in the way of writing either the libretto or the score of Ernani but, by January, the situation and the subject had changed. Bellini wrote that "[Romani] is now writing La sonnambula, ossia I Due Fidanzati svezzeri...It must go on stage on 20 February at the latest."