Louis Gallet was a French writer of operatic libretti, plays, romances, memoirs, pamphlets, and innumerable articles, who is remembered above all for his adaptations of fiction—and Scripture— to provide librettos of cantatas and opera, notably by composers Georges Bizet, Camille Saint-Saëns and Jules Massenet.

By day Gallet supported himself by a minor post in the Administration of Assistance to the Poor and positions, first as treasurer then as general administrator, at the Beaujon hospital, Paris, and other hospitals.

In 1871 Camille du Locle, the manager of the Paris Opéra-Comique offered to produce a one-act work of Camille Saint-Saëns. He proposed as collaborator Louis Gallet, whom Saint-Saëns did not know, and the result was the slight piece La princesse jaune notable as the first japonerie on the operatic stage, Japan having only very recently been opened to Western trade and the first Japanese woodblock prints having been seen in Paris only two years previously. The two worked together harmoniously for years, and it was Saint-Saëns who recommended Gallet as music critic for the Nouvelle Revue, though he was not a musician.

For Massenet he first provided a libretto for the oratorio Marie-Magdeleine (1872) which proved to be Massenet's first major success and the first of his four dramatic oratorios.

Georges Bizet's one-act opera Djamileh to Gallet's libretto premiered successfully, 22 May 1872 at the Opéra-Comique, Paris), but two other Bizet operas by Gallet and Edouard Blau remained incomplete at Bizet's untimely death in 1875: La coupe du roi de Thulé (1869) and a five-act Don Rodrigue (1873).

In his libretto for Massenet's Thaïs he employed an unrhymed free verse that he termed, in Parnassien fashion, poésie melique which, like its classical Greek predecessors, was designed for a declamation with accompaniment (melodrama). In Gallet's hands declamation rose by degrees into a freely-structured aria that was raised above the level of prose by its sonorities and syntactical patterns, formulas that were finely suited to the musical techniques of both Saint-Saëns and Massenet.