Benedetto Giacomo Marcello (Italian: [beneˈdetto marˈtʃɛllo]; 31 July or 1 August 1686 – 24 July 1739) was an Italian composer, writer, advocate, magistrate, and teacher.

Born in Venice, Benedetto Marcello was a member of the noble Marcello family and in his compositions, he is frequently referred to anonymously as Patrizio Veneto (Venetian patrician, i.e. aristocrat). Although he was a music student of Antonio Lotti and Francesco Gasparini, his father wanted Benedetto to devote himself to law. Benedetto managed to combine a life in law and public service with one in music. In 1711 he was appointed a member of the Council of Forty (in Venice's central government), and in 1730 he went to Pola as Provveditore (district governor). Due to his health having been "impaired by the climate" of Istria, Marcello retired after eight years in the capacity of Camerlengo (chamberlain) to Brescia where he died of tuberculosis in 1739.

Benedetto Marcello was the brother of Alessandro Marcello, also a notable composer. On 20 May 1728 Benedetto Marcello married his singing student Rosanna Scalfi in a secret ceremony. However, as a nobleman, his marriage to a commoner was unlawful; after his death, the marriage was declared null by the state. Rosanna was unable to inherit his estate and filed suit in 1742 against Benedetto's brother Alessandro, seeking financial support.

Marcello composed a variety of music including considerable church music, oratorios, hundreds of solo cantatas, duets, sonatas, concertos and sinfonias. Marcello was a younger contemporary of Antonio Vivaldi in Venice and his instrumental music enjoys a Vivaldian flavour.

As a composer, Marcello was best known in his lifetime and is now still best remembered for his Estro poetico-armonico (Venice, 1724–27), a musical setting for voices, figured bass (a continuo notation), and occasional solo instruments, of the first fifty Psalms, as paraphrased in Italian by his friend G. Giustiniani. They were much admired by Charles Avison, who with John Garth brought out an edition with English words (London, 1757). Estro poetico-armonico also represents an important contribution to the history of Jewish liturgical music. Eleven of the Psalms are set to melodies that Marcello apparently transcribed while attending services at several Venetian synagogues. The eleven melodies – six from the Ashkenazic tradition, and five from the Sephardic tradition – are among the earliest notated sources of Jewish liturgy, preceded only by Salamone Rossi's Hashirim Asher L’Shlomo. Perhaps the best known of these melodies is an Ashkenazic melody for Ma'oz Tzur.

The library of the Brussels Conservatoire possesses some interesting volumes of chamber cantatas composed by Marcello for his mistress. Although Benedetto Marcello wrote an opera called La Fede riconosciuta and produced it in Vicenza in 1702, he had little sympathy with this form of composition, as evidenced in his writings (see below).

Benedetto Marcello's music is "characterized by imagination and a fine technique and includes both counterpoint and progressive, galant features".(Grove 1994).

With the poet Antonio Schinella Conti he wrote a series of experimental long cantatas – a duet, Il Timoteo, then five monologues, Cantone, Lucrezia, Andromaca, Arianna abandonnata, and finally Cassandra.