Agazzari was a cadet of a noble family of Siena, and born on Dec 2, 1578. He is said, but on rather slight authority to have passed the first years of his professional life in the service of the Emperor Matthias. About 1600 he came to Rome, where he was chosen Maestro di Cappella at the German College (after Anerio's retirment in 1609), at the church of S. Apollinaris, and subsequently at the Seminario Romano. An intimacy grew up between him and the well known Viadana, of Mantua, and he was one of the earliest adopters of the figured bass. In the preface to the fourth book of his 'Sacrae Cantiones' (Zanetti, Rome, 1606), he gives some instructions for its employment.

In 1630 he returned to Siena, and became Maestro of its cathedral, a post which he retained till his death, probably April 10, 1640. Agazzari was a member of the Academy of the Armonici Intronati. His publications are numerous, and consist of Masses, Madrigals (1596, 1600, 1602, 1607, 1608), Motets, Psalms, Magnificats, Litanies etc., republished in numerous editions at Rome, Milan, Venice, Antwerp, Frankfort, and elsewhere.

His one substantive contribution to the scientific literature of music is a little work of only sixteen quarto pages, entitled 'La Musica Ecclesiastica, dove si contiene la vera diffinizione della Musica come Scienza non piu veduta e sua noblita' (Siena, 1638); the object of which is to determine how church music should best conform itself to the Resolution of the Council of Trent. On the authority of Pitoni, a pastoral drama, entitled 'Eumelio,' has been ascribed to Agazzari. It was undoubtedly performed at Rom ein 1606 and printed by Amadino at Venice in that year; but no author's name is affixed either to music or libretto.

A short motet by Agazzari is given by Proske in the 'Musica Divina'.