Ruggero (or Ruggiero) Giacomo Maria Giuseppe Emmanuele Raffaele Domenico Vincenzo Francesco Donato Leoncavallo (Italian pronunciation: [rudˈdʒɛːro leoŋkaˈvallo]; 23 April 1857 – 9 August 1919) was an Italian opera composer. His two-act work Pagliacci remains one of the most popular works in the repertory, appearing as number 20 on the Operabase list of the most-performed operas worldwide.

The son of a judge, Leoncavallo was born in Naples on 23 April 1857. As child he moved with his father in the town of Montalto Uffugo in Calabria where Leoncavallo lived during his adolescence. He later returned to Naples and was educated at the city's San Pietro a Majella Conservatory. After some years spent teaching and in ineffective attempts to obtain the production of more than one opera, he saw the enormous success of Pietro Mascagni's Cavalleria rusticana in 1890, and he wasted no time in producing his own verismo hit, Pagliacci. (According to Leoncavallo, the plot of this work had a real-life origin: he claimed it derived from a murder trial, in Montalto Uffugo, over which his father had presided.)

Pagliacci was performed in Milan in 1892 with immediate success; today it is the only work by Leoncavallo in the standard operatic repertory. Its most famous aria "Vesti la giubba" ("Put on the costume" or, in the better-known older translation, "On with the motley") was recorded by Enrico Caruso and laid claim to being the world's first record to sell a million copies (although this is probably a total of Caruso's various versions of it made in 1902, 1904 and 1907).

The next year his I Medici was also produced in Milan, but neither it nor Chatterton (belatedly produced in 1896)—both early works—obtained much lasting favour. Much of Chatterton, however, was recorded by the Gramophone Company (later HMV) as early as 1908, and remastered on CD almost 100 years later by Marston Records. Leoncavallo himself conducts the performance or at very least supervises the production.

It was not until Leoncavallo's La bohème was performed in 1897 in Venice that his talent obtained public confirmation. However, it was outshone by Puccini's opera of the same name and on the same subject, which was premiered in 1896. Two tenor arias from Leoncavallo's version are still occasionally performed, especially in Italy.

Subsequent operas by Leoncavallo were in the 1900s: Zazà (the opera of Geraldine Farrar's famous farewell performance at the Metropolitan Opera), and 1904's Der Roland von Berlin. He had a brief success with Zingari which premiered in Italian in London in 1912, with a long run at the Hippodrome Theatre. Zingari also reached the United States but soon disappeared from the repertoire.

After a series of operettas, Leoncavallo tried for one last serious effort (Edipo Re), but he died before he could finish the orchestration, which was completed by Giovanni Pennacchio. From the 1970s the opera has had a number of revivals, both as concert performances (including Rome 1972, Concertgebouw (Amsterdam) 1977 and Konzerthaus, Vienna 1998) as well as fully staged productions at the Teatro Regio, Turin, in 2002 and the Thessaloniki Opera 2008. In Edipo Re (a short one act work) the composer uses exactly the same melody for the final scene "Miei poveri fior, per voi non più sole..." (with the blinded Edipo) as he had for the act 4 soprano aria from Der Roland von Berlin. It has been assumed (see The New Grove Dictionary of Opera) that Leoncavallo left the opera more or less complete (except for the orchestration), but Pennacchio may have had to do more and may have "filled in the gaps" using Leoncavallo's earlier music. Unusually, Leoncavallo did not write the libretto. The libretto for Edipo Re was written by Giovacchino Forzano, who also wrote Il piccolo Marat for Pietro Mascagni and two of the one-act operas for Puccini's Il trittico.

Little or nothing from Leoncavallo's other operas is heard today, but the baritone arias from Zazà were great concert and recording favourites among baritones and Zazà as a whole is sometimes revived, as is his La bohème. The tenor arias from La bohème remain recording favorites.

Leoncavallo also composed songs, most famously Mattinata, which he wrote for the Gramophone Company (which became HMV) with Caruso's unique voice in mind. On 8 April 1904, Leoncavallo accompanied Caruso at the piano as they recorded the song. On 8 December 1905 he recorded five of his own pieces for the reproducing piano Welte-Mignon.

Leoncavallo was the librettist for most of his own operas. Many considered him the greatest Italian librettist of his time after Boito. Among Leoncavallo's libretti for other composers is his contribution to the libretto for Puccini's Manon Lescaut.

Ruggero Leoncavallo died in Montecatini Terme, Tuscany, on 9 August 1919.