La fille du régiment (French pronunciation: [la fij dy ʁeʒimɑ̃], The Daughter of the Regiment) is an opéra comique in two acts by Gaetano Donizetti, set to a French libretto by Jules-Henri Vernoy de Saint-Georges and Jean-François Bayard. It was first performed on 11 February 1840 by the Paris Opéra-Comique at the Salle de la Bourse.
Donizetti wrote the opera while living in Paris between 1838 and 1840 and preparing a revised version of his then-unperformed Italian opera, Poliuto, as Les martyrs for the Paris Opéra. Since Martyrs was delayed, the composer had time to write the music for La fille du régiment, his first opera set to a French text, and to stage the French version of Lucia di Lammermoor, Lucie de Lammermoor.
La fille du régiment quickly became a popular success partly because of the famous aria "Ah! mes amis, quel jour de fête!", which requires the tenor to sing no fewer than eight high Cs – a frequently sung ninth is not written. La figlia del reggimento, a slightly different Italian-language version (in translation by Calisto Bassi), was adapted to the tastes of the Italian public.