Miserere, full name "Miserere mei, Deus" (Latin: "Have mercy on me, O God") by Italian composer Gregorio Allegri, is a setting of Psalm 51 (50) composed during the reign of Pope Urban VIII, probably during the 1630s, for use in the Sistine Chapel during matins, as part of the exclusive Tenebrae service on Wednesday and Friday of Holy Week. The service would start usually around 3AM, and during the ritual, candles would be extinguished, one by one, until one remained alight and hidden. Allegri composed his setting of the Miserere for the final act within the first lesson of the Tenebrae service.

It was the last of twelve falsobordone Miserere settings composed and chanted at the service since 1514 and the most popular: at some point, it became forbidden to transcribe the music and it was allowed to be performed only at those particular services, adding to the mystery surrounding it. Writing it down or performing it elsewhere was punishable by excommunication. The setting that escaped from the Vatican is actually a conflation of verses set by Gregorio Allegri around 1638 and Tommaso Bai (also spelled "Baj"; 1650–1718) in 1714.

The Miserere is written for two choirs, one of five and one of four voices, and is an example of Renaissance polyphony surviving to the present day. One of the choirs sings a simple version of the original Miserere chant; the other, spatially separated, sings an ornamented "commentary" on this. The piece is an example of the stile antico or prima pratica; however, its constant use of the dominant seventh chord and its emphasis on polychoral techniques certainly put it out of the range of prima pratica; a more accurate comparison would be to the works of Giovanni Gabrieli.

Three authorized copies of the work were distributed prior to 1770 – to the Holy Roman Emperor Leopold I, to the King of Portugal, and to Padre (Giovanni Battista) Martini. However, none of them succeeded in capturing the beauty of the Miserere as performed annually in the Sistine Chapel. According to the popular story (backed up by family letters), the fourteen-year-old Mozart was visiting Rome, when he first heard the piece during the Wednesday service. Later that day, he wrote it down entirely from memory, returning to the Chapel that Friday to make minor corrections. Some time during his travels, he met the British historian Dr Charles Burney, who obtained the piece from him and took it to London, where it was published in 1771. Once the piece was published, the ban was lifted; Mozart was summoned to Rome by the Pope, only instead of excommunicating the boy, the Pope showered praises on him for his feat of musical genius. The work was also transcribed by Felix Mendelssohn in 1831 and Franz Liszt, and various other 18th and 19th century sources survive. Since the lifting of the ban, Allegri's Miserere has become one of the most popular a cappella choral works now performed.

Burney's edition did not include the ornamentation that made the work famous. The original ornamentations were Renaissance techniques that preceded the composition itself, and it was these techniques that were closely guarded by the Vatican. Few written sources (not even Burney's) showed the ornamentation, and it was this that created the legend of the work's mystery. However, the Roman priest Pietro Alfieri published an edition in 1840 with the intent of preserving the performance practice of the Sistine choir in the Allegri and Bai compositions, including ornamentation.