Agostino Steffani (25 July 1654 – 12 February 1728) was an Italian ecclesiastic, diplomat and composer.
Steffani was born at Castelfranco Veneto. At a very early age he was admitted as a chorister at San Marco, Venice. In 1667 the beauty of his voice attracted the attention of Count Georg Ignaz von Tattenbach, by whom he was taken to Munich, where his education was completed at the expense of Ferdinand Maria, Elector of Bavaria, who appointed him Churfürstlicher Kammer- und Hofmusikus and granted him a liberal salary. After receiving instruction from Johann Kaspar Kerll, in whose charge he lived, he was sent in 1673 to study in Rome, where Ercole Bernabei was his master, and among other works he composed six motets, the original manuscripts of which are now in the Fitzwilliam Museum at Cambridge.
On his return to Munich in 1674 he published his first work, Psalmodia vespertina, a part of which was reprinted in Giovanni Battista Martini's Saggio di contrappunto in 1774. In 1675 he was appointed court organist. The date when he was ordained priest, with the title of Abbate of Lepsing, is not precisely known. His ecclesiastical status did not prevent him from turning his attention to the stage, for which, at different periods of his life, he composed work which undoubtedly exercised a potent influence upon the dramatic music of the period. Of his first opera, Marco Aurelio, written for the carnival and produced at Munich in 1681, the only copy known to exist is a manuscript score preserved in the royal library at Buckingham Palace. It was followed by Solone in 1685, by Audacia e rispetto, Prerogative d'amore and Servio Tulio in 1686, by Alarico in 1687, and by Niobe, regina di Tebe in 1688.