Arrigo Boito (24 February 1842 – 10 June 1918), aka Enrico Giuseppe Giovanni Boito, pseudonym Tobia Gorrio, was an Italian poet, journalist, novelist and composer, best known today for his libretti, especially those for Giuseppe Verdi's operas Otello and Falstaff, and his own opera Mefistofele.

Born in Padua, the son of Silvestro Boito, an Italian painter of miniatures and his wife, a Polish countess, Józefina Radolińska, Boito studied music at the Milan Conservatory with Alberto Mazzucato until 1861. In 1866 he fought under Giuseppe Garibaldi in the Seven Weeks War in which the Kingdom of Italy and Prussia fought against Austria, after which Venice was ceded to Italy.

His only finished opera, Mefistofele, based on Goethe's Faust, was given its first performance on 5 March 1868, at La Scala, Milan. The premiere, which he conducted himself, was badly received, provoking riots and duels over its supposed "Wagnerism", and it was closed by the police after two performances. Verdi commented, "He aspires to originality but succeeds only at being strange." Boito withdrew the opera from further performances to rework it, and it had a more successful second premiere, in Bologna on 10 April 1875. Boito's revised and drastically cut version also changed Faust from a baritone to a tenor, and it is still frequently performed and recorded today.

Other than Mefistofele, Boito wrote very little music. He completed (but later destroyed) another opera, Ero e Leandro, and left incomplete a further opera, Nerone, which he had been working at, on and off, between 1877 and 1915; excluding its last act, for which Boito left only a few sketches, Nerone was finished after his death by Arturo Toscanini and Vincenzo Tommasini and premiered at La Scala, 1924. Mefistofele is the only work of his performed with any regularity today. The Prologue to the opera, set in Heaven, is a favorite concert piece. He also left a Symphony in A minor in manuscript.

Boito's literary powers never dried up. As well as writing the libretti for his own operas, he wrote them for other composers. As "Tobia Gorrio" (an anagram of his name) he provided the libretto for Amilcare Ponchielli's La Gioconda. His rapprochement with Verdi, whom he had offended in a toast shortly after they had collaborated on Verdi's Inno delle Nazioni ("Anthem of the Nations", London, 1862), was effected by the music publisher Giulio Ricordi. Boito successfully revised the libretto for Verdi's unwieldy Simon Boccanegra, which then premiered to great acclaim in 1881. With that, their mutual friendship and respect blossomed and, though Verdi's projection for an opera based on King Lear never came to anything, Boito provided subtle and resonant libretti for Verdi's last masterpieces, Otello (1887) and Falstaff (1893). When Verdi died, Boito was there at his bedside.

Boito succeeded Giovanni Bottesini as director of the Parma Conservatory after the latter's death in 1889 and held the post until 1897. He received the honorary degree of doctor of music from the University of Cambridge in 1893. He died in Milan and was interred there in the Cimitero Monumentale.

A memorial concert was given in his honor at La Scala in 1948. The orchestra was conducted by Arturo Toscanini. Recorded in very primitive sound, the concert has been issued on CD.

Camillo Boito, Arrigo's older brother, was an Italian architect and engineer, and a noted art critic, art historian and novelist.