Mario Castelnuovo-Tedesco (April 3, 1895 – March 16, 1968) was an Italian Jewish composer. Born in Florence, he was descended from a prominent banking family that had lived in the city since the expulsion of the Jews from Spain in 1492.

Castelnuovo was first introduced to the piano by his mother, and he composed his first pieces when he was just nine years old. After completing a degree in piano in 1914, he began studying composition under renowned Italian composer Ildebrando Pizzetti , and receiving a diploma in composition in 1918. He soon came to the attention of composer and pianist Arturo Casella , who included the young Castelnuovo's work in his repertoire. Casella also ensured that Castelnuovo's works would be included in the repertoires of the Societa Nazionale di Musica (later the Societa Italiana di Musica Moderna ), granting him exposure throughout Europe as one of Italy's up-and-coming young composers. Works by him were included in the first festival of the International Society of Contemporary Music , held in Salzburg, Austria, in 1922.

In 1926, Castelnuovo premiered his opera, La Mandragoa, based on a play by Niccolò Machiavelli. It was the first of his many works inspired by great literature, and which included interpretations of works by Aeschylus, Virgil, John Keats, William Wordsworth, Walt Whitman, and especially William Shakespeare. Another major source of inspiration for him was his Jewish heritage, most notably the Bible and Jewish liturgy. His Violin Concerto no. 2 (1931), written at the request of Jascha Heifetz, was also an expression of his pride in his Jewish origins, or as he described it, the "splendor of past days," in the face of rising anti-Semitism that was sweeping across much of Europe.

At the 1932 festival of the International Society of Contemporary Music, held in Venice, Castelnuovo first met the famous Spanish guitarist Andrés Segovia. The meeting inspired Castelnuovo to write his Concerto in D for Guitar, the first of almost one hundred compositions for that instrument, which earned him the reputation as one of the foremost composers for the guitar in the twentieth century.

The following year the Italian fascist government developed a program toward the arts, which were viewed as a tool for propaganda and promotion of racial ideas. Even before Mussolini officially adopted the Manifesto of Race in 1938, Castelnuovo was banned from the radio and performances of his work were cancelled. The new racial laws, however, convinced him that he should leave Italy. He wrote to Arturo Toscanini, the former musical director of La Scala, who has left Italy in 1933, explaining his plight, and Toscanini responded by promising to sponsor him as an immigrant in the United States. Castelnuovo left Italy in 1939, shortly before the outbreak of World War II.

Like many artists who fled fascism, Castelnuovo ended up in Hollywood, where, with the help of Jascha Heifetz, he landed a contract with MGM as a film composer. Over the next fifteen years, he worked on scores for some 200 films there and at the other major film studios. He was a significant influence on other major film composers, inluding Henry Mancini, Jerry Goldsmith, Nelson Riddle, John Williams, and André Previn. HIs relationship to Hollywood was ambiguous: later in life he attempted to deny the influence that it had on his own work, but he also believed that it was an essentially American artform, much as opera was European.

In the United States, Castelnuovo also composed new operas and works based on American poetry, Jewish liturgy, and the Bible. He died in 1968.