Alfred Cellier (1 December 1844 – 28 December 1891) was an English composer, orchestrator and conductor.

In addition to conducting and music directing the original productions of several of the most famous Gilbert and Sullivan works and writing the overtures to some of them, Cellier conducted at many theatres in London, New York and on tour in Britain, America and Australia. He composed over a dozen operas and other works for the theatre, as well as for orchestra, but his 1886 comic opera, Dorothy, was by far his most successful work. It became the longest-running piece of musical theatre in the nineteenth century.

Cellier was born in South Hackney, London, the second child and eldest son of Arsène Cellier, a language teacher from France, and his wife Mary Ann Peterine, formerly Peacock, née Thomsett. He was educated at the grammar school in Hackney. In the 1850s he was a chorister at the Chapel Royal, St. James's, under the Rev. Thomas Helmore, where Arthur Sullivan was one of his schoolmates. Cellier later married Harriet Emily. Cellier's brother, François, also became a conductor.

Cellier's first appointments were as organist at All Saints' Church, Blackheath and as conductor of the Belfast Philharmonic Society (both in 1862). In 1866 he succeeded Dr. Chipp as organist and director of the Ulster Hall concerts, Belfast, at the same time acting as conductor of the Belfast Philharmonic Society. In 1868 he returned to London as organist of St Alban's Church, Holborn. In January 1871, Cellier became the first conductor and music director at the Royal Court Theatre in London. From 1871 to 1875 he was conductor and musical director at the Prince's Theatre in Manchester.

During this period he composed many comic operas and operettas, the first of which was Charity Begins at Home (1872 at the Gallery of Illustration), with the librettist B. C. Stephenson. The piece was a success and played more than 200 times. A reviewer in the London and Provincial Entr'Acte wrote that the music "is unexpectedly apropos and pretty ... and we have no hesitation in saying that Mr. Alfred Cellier's melodies will live even after the long life [of the production] shall have come to an end." He achieved a favourable outcome with The Sultan of Mocha, produced at Prince's Theatre, Manchester, in 1874 and revived in London in 1876 and 1887 (with a new libretto) and in New York in 1880, among others. Most of his early works for the theatre, including Dora's Dream (1873), Topsyturveydom (1874, with a libretto by W. S. Gilbert), The Tower of London (1875, Manchester), Nell Gwynne (1876), Two Foster Brothers (1877, St. George's Hall, with a libretto by Gilbert Arthur à Beckett), and Bella Donna (1878), had only modest success. Cellier also wrote numerous separate songs and composed for orchestra (including his Suite Symphonique (1978)) and the piano; his Danse pompadour achieved particular popularity.

In December 1877 Cellier joined the D'Oyly Carte company as musical director at the Opera Comique in London. There he conducted The Sorcerer (1877), H.M.S. Pinafore (1878, for which he wrote the overture, based on themes from the opera), Trial by Jury (1878), George Grossmith's Cups and Saucers (1878–79), and three of his own one-act works: Dora's Dream (1877–78 revival), The Spectre Knight (1878), and After All! (1878–79). Cellier was conducting the performance of Pinafore during which the partners of The Comedy Opera Company attempted to repossess the set, and he was noted for his attempts to calm the audience during the fracas. His brother, François, succeeded him as musical director at the Opera Comique in 1879. Alfred Cellier was a conductor of a series of promenade concerts at the Queen's Theatre, Long Acre and, in 1878–1879 he was joint conductor, with Sullivan, of the Covent Garden Promenade Concerts.

In 1879, he travelled with Gilbert, Sullivan, and Carte to America, where he acted as conductor for Pinafore and The Pirates of Penzance, with Carte's first American touring company. Cellier prepared the overture to Pirates using Sullivan's music from the rest of the score. Back in London in July 1880, he directed the music at the Opera Comique for Pirates and another of his own pieces with Desprez, In the Sulks. In April 1881, he left the D'Oyly Carte company, ceding the baton to his brother. Cellier composed a three-act grand opera, Pandora, a version of Longfellow's The Masque of Pandora (with a libretto by B. C. Stephenson) that was presented in Boston, Massachusetts, in 1881. Reviews of the premiere were mixed, praising the "gracefulness" of the music, but finding the opera lacking in dramatic intensity; it closed after two weeks. Cellier remained in America to conduct the Comly-Barton Opera Company on tour in the American West. Later that year he was music director of D'Oyly Carte's New York and touring productions of Billee Taylor (1882), Les Manteaux Noirs and Rip Van Winkle (both in the fall of 1882), and Iolanthe (1882–83), for the latter of which he prepared the New York overture.

In 1883, Cellier's setting of Gray's Elegy, in the form of a cantata, was produced at the Leeds music festival. In 1883, Cellier left the D'Oyly Carte company, but he was back for brief periods as music director with D'Oyly Carte's touring companies for Princess Ida (1884) and The Mikado (1885). In 1885, also, Cellier composed incidental music for a production of As You Like It. He composed two more companion pieces that had Savoy Theatre premieres, both with libretti by Desprez: The Carp (performed with The Mikado and Ruddigore in 1886–87), and Mrs. Jarramie's Genie (composed together with his brother François, which played together with several different operas at the Savoy between 1887 and 1889.