Marco da Gagliano (1 May 1582 – 25 February 1643) was an Italian composer of the early Baroque era. He was important in the early history of opera and the development of the solo and concerted madrigal.
He was born in Florence and lived most of his life there. After early study both with a religious confraternity and with Luca Bati, he was employed beginning in 1602 at the church of San Lorenzo for six years as a singing instructor. In 1607 he went to Mantua, where he wrote music for the Gonzaga family, including his impressive operatic setting of La Dafne, and in 1609 returned to Florence to become maestro di cappella at the Compagnia dell'Arcangelo Raffaello, the organisation at which he had received his boyhood musical training. Later that same year the Medici made him maestro di cappella of their court, a position he held for 35 years.
Gagliano wrote an enormous quantity of music, both sacred and secular, for the Medici, and in addition he was a singer and instrumentalist who entertained them privately. His works include fourteen published operas of which two survive, La Flora (1628) set to a libretto by Andrea Salvadori and La Dafne (1608). La Dafne was praised as the best setting of the libretto by Rinuccini—even by Jacopo Peri, the first to write an opera on the text. Meanwhile, Gagliano or somebody else changed for him Rinuccini's poetry so strongly that sometimes it is impossible to recognize traces of the original. Peri indicated that Gagliano's way of setting text to music came closer to actual speech than any other, therefore accomplishing the aim of the Florentine Camerata of decades before, who sought to recapture that (supposed) aspect of ancient Greek music.

Other music by Gagliano includes secular monodies and numerous madrigals. While the monody was a Baroque stylistic innovation, most of the madrigals are a cappella, and written in a style reminiscent of the late Renaissance (in the first decades of the 17th century, the continuo madrigal was becoming predominant, for example in the works of Monteverdi). This mix of progressive and conservative trends can be seen throughout his music: some of his sacred music is a cappella, again in the prima prattica style of the previous century, while other pieces show influence of the Venetian School.

Gagliano was extremely influential in his time, as could be expected of the Medici's own appointed head of all musical activities at their court; however his popularity waned after his death, and his music has since been overshadowed by contemporaries such as Monteverdi.