Venetian composer and writer on music, the most celebrated music theorist of the mid-16th century.

Zarlino took deacon's orders in 1541 and studied music under Adriaan Willaert at St. Mark's in Venice, where in 1565 he became music director. Although he was esteemed as a composer, few of his works survive. He was offered the bishopric of Chioggia in 1583, but the Venetian senate persuaded him to remain in Venice.

Zarlino's first treatise, Istitutioni harmoniche (1558), brought him rapid fame. It gives a shrewd account of musical thinking during the first half of the 16th century, and Zarlino's thoughts on tuning, chords, and modes anticipate 17th- and 18th-century developments. He discussed the tuning of the first four intervals of the scale (tetrachord), espousing a system that proved reliable in subsequent practice. He stressed the importance of the major and minor third and discussed the nature of the major and minor triads, anticipating the theories of later writers such as Jean-Philippe Rameau. He renumbered the medieval modes, placing the Ionian mode (corresponding to the modern major scale) first. He also gave one of the two earliest accounts of double counterpoint and offered detailed advice on the setting of words to music. His Dimostrationi harmoniche (1571) consists of five dialogues between Willaert and four friends; it amplifies much of the material he had set forth in the Istitutioni.

Zarlino's theories were violently attacked by Vincenzo Galilei (Galilei, Vincenzo), his former pupil and a member of the Florentine Camerata, a group influential in the evolution of opera. Zarlino replied with Sopplimenti musicali (1588) and collected his works into a complete edition in 1589. The Sopplimenti reinforces and develops his previous theories. One passage suggests equally tempered tuning for the lute (in advance of 18th-century experiments with equal temperament on keyboard instruments); another gives valuable descriptions of early organs. Zarlino strongly decried the monodic recitative of the Camerata, insisting that music has its own laws and should not abandon them in order to imitate the spoken word.