Gregorio Allegri (c. 1582 – 7 February 1652) was an Italian composer of the Roman School and brother of Domenico Allegri; he was also a priest and a singer. He was born and died in Rome.
He studied music as a puer (boy chorister) at San Luigi dei Francesi, under the maestro di capella Giovanni Bernardino Nanino, brother of Giovanni Maria Nanino. Being intended for the Church, he obtained a benefice in the cathedral of Fermo. Here he composed a large number of motets and other sacred music, which, being brought to the notice of Pope Urban VIII, obtained for him an appointment in the choir of the Sistine Chapel at Rome as a contralto. He held this from 6 December 1629 until his death. As Andrea Adami wrote, Allegri was regarded as singularly pure and benevolent.
Among Allegri's musical compositions were two volumes of concerti for five voices published in 1618 and 1619; two volumes of motets for six voices published in 1621; an edition of a four-part sinfonia; five masses; two settings of the Lamentations of Jeremiah; and numerous motets which were not published in his lifetime. He was one of the earliest composers for stringed instruments, and Athanasius Kircher has given one specimen of this class of his works in his Musurgia. Most of Allegri's published music, especially the instrumental music, is in the progressive early Baroque concertato style. However, his work for the Sistine Chapel is descended from the Palestrina style, and in some cases strips even this refined, simple style of all ornament. He is credited with the earliest string quartet.