Pierre Benjamin Monteux (pronounced [pjɛʁ mɔ̃.tø]; 4 April 1875 – 1 July 1964) was a French (later American) conductor. After violin and viola studies, and a decade as an orchestral player and occasional conductor, he began to receive regular conducting engagements in 1907. He came to prominence when, for Sergei Diaghilev's Ballets Russes company between 1911 and 1914, he conducted the world premieres of Stravinsky's The Rite of Spring and other prominent works including Petrushka, The Nightingale, Ravel's Daphnis et Chloé, and Debussy's Jeux. Thereafter he directed orchestras around the world for more than half a century.

From 1917 to 1919 Monteux was the principal conductor of the French repertoire at the Metropolitan Opera in New York. He conducted the Boston Symphony Orchestra (1919–24), Amsterdam Concertgebouw Orchestra (1924–34), Orchestre Symphonique de Paris (1929–38) and San Francisco Symphony (1936–52). In 1961, aged eighty-six, he accepted the chief conductorship of the London Symphony Orchestra, a post which he held until his death three years later. Although he was known for his performances of the French repertoire, his chief love was the music of German composers, above all Brahms. He disliked recording, finding it incompatible with spontaneity, but he nevertheless made a substantial number of records.

Monteux was well known as a teacher. In 1932 he began a conducting class in Paris, which he developed into a summer school that was later moved to his summer home in Les Baux in the south of France. After moving permanently to the US in 1942 and taking American citizenship, he founded a school for conductors and orchestral musicians in Hancock, Maine. Among his students in France and America who went on to international fame were Lorin Maazel, Igor Markevitch, Neville Marriner, Seiji Ozawa, André Previn and David Zinman. The school in Hancock has continued since Monteux's death.

Pierre Monteux was born in Paris, the third son and the fifth of six children of Gustave Élie Monteux, a shoe salesman, and his wife, Clémence Rebecca née Brisac. The Monteux family was descended from Sephardic Jews who settled in the south of France. The Monteux ancestors included at least one rabbi, but Gustave Monteux and his family were not religious. Among Monteux's brothers were Henri, who became an actor, and Paul (1862-1928), who became a conductor of light music under the name Paul Monteux-Brisac. Gustave Monteux was not musical, but his wife was a graduate of the Conservatoire de Musique de Marseille and gave piano lessons. Pierre took violin lessons from the age of six.

When he was nine years old Monteux was admitted to the Conservatoire de Paris. He studied the violin with Jules Garcin and Henri Berthelier, composition with Charles Lenepveu, and harmony and theory with Albert Lavignac. His fellow violin students included George Enescu, Carl Flesch, Fritz Kreisler and Jacques Thibaud. Among the piano students at the Conservatoire was Alfred Cortot, with whom he developed a lifelong friendship. At the age of twelve, Monteux organised and conducted a small orchestra of Conservatoire students to accompany Cortot in performances of concertos in and around Paris. He attended the world premiere of César Franck's Symphony in February 1889. From 1889 to 1892, while still a student, he played in the orchestra of the Folies Bergère; he later said to George Gershwin that his rhythmic sense was formed during the experience of playing popular dance music there.

At the age of fifteen, while continuing his violin studies, Monteux took up the viola. He studied privately with Benjamin Godard, with whom he performed in the premiere of Saint-Saëns's Septet, with the composer at the keyboard. Monteux joined the Geloso Quartet as violist; he played many concerts with them, including a performance of Fauré's Second Piano Quartet with the composer at the piano. On another occasion he was the violist in a private performance of a Brahms quartet given before the composer in Vienna. Monteux recalled Brahms's remark, "It takes the French to play my music properly. The Germans all play it much too heavily." Monteux remained a member of the Geloso Quartet until 1911. With Johannes Wolff and Joseph Hollman he also played chamber music for Grieg. Years later, in his seventies, Monteux deputised with the Budapest Quartet without rehearsal or score; asked by Erik Smith if he could write out the parts of the seventeen Beethoven quartets, he replied, "You know, I cannot forget them."

In 1893, when he was eighteen, Monteux married a fellow student, the pianist Victoria Barrière. With her he played the complete Beethoven violin sonatas in public. Neither family approved of the marriage; although the Monteux family were not religious, both they and the Roman Catholic Barrières were doubtful about an inter-religious marriage; furthermore, both families thought the couple too young to marry. There were a son and a daughter from the union.

During his formative years Monteux belonged to a group which toured with the Casadesus family of musicians and the pianist Alfredo Casella. The combination played supposed "ancient pieces", allegedly discovered in libraries by one or other of the Casadesus family; Marius Casadesus later revealed that he or his brother Henri had written the music. While still a student, in 1893 Monteux was successful in the competition for the chair of first viola of the Concerts Colonne, of which he became assistant conductor and choirmaster the following year. This gave him a link via the orchestra's founder, Édouard Colonne, to Berlioz. Colonne had known Berlioz, and through the older conductor Monteux was able to mark his scores with notes based on the composer's intentions. He was also employed on a freelance basis at the Opéra-Comique, where he continued to play from time to time for several years; he led the viola section at the 1902 premiere of Pelléas et Mélisande under the baton of André Messager. In 1896 he graduated from the Conservatoire, sharing first prize for violin with Thibaud.