Sergei Pavlovich Diaghilev (March 19, 1872 - August 19, 1929), often known as Serge, was a Russian ballet impressario and founder of the Ballets Russes from which many famous dancers and choreographers would later arise.

Born in Perm, Russia, in 1910 Diaghilev organised the first of 20 ballet seasons in Paris, France. This period was of tremendous importance to the development of ballet as a performing art in theatre.

In 1910 Diaghilev started a series of ballet seasons in Paris with his Ballets Russes, with famous dancers like Anna Pavlova and Vaslav Nijinsky. Diaghilev used ballet music which he commissioned from composers such as Claude Debussy (Jeux, 1913), Maurice Ravel (Daphnis et Chloé, 1912), Erik Satie (Parade, 1917), Richard Strauss (Josephs-Legende, 1914), Sergei Prokofiev (Ala and Lolly, rejected by Diaghilev and turned into the Scythian Suite, and Chout, 1915), Ottorino Respighi (La Boutique fantasque, 1918), Francis Poulenc (Les Biches, 1923) and others. His choreographer Mikhail Fokine often adapted the music for ballet. The artistic director for the Ballets Russes was Leon Bakst, whose connection with Diaghilev extended back to 1898, when he, Diaghilev and Alexander Benois founded the avant-garde group 'World of Art' (Mir Iskusstva). Together they developed a more complicated form of ballet with show-elements intended to appeal to the general public, rather than solely the aristocracy. The exotic appeal of the Ballets Russes had an effect on Fauve painters and the nascent Art Deco style.

Perhaps Diaghilev's most notable composer collaborator, however, was Igor Stravinsky. Diaghilev heard Stravinsky's early orchestral works Fireworks and Scherzo Fantastique, and was impressed enough to ask Stravinsky to arrange some pieces by Frederic Chopin for the Ballets Russes. In 1910, he commissioned his first score from Stravinsky, The Firebird. Petrushka (1911) and The Rite of Spring (1913) followed shortly afterwards, and the two also worked together on Pulcinella (1920) and Les Noces (1923).

The end of the nineteenth century brought a development in the handling of tonality, harmony, rhythm and meter towards more freedom. Until that time, rigid harmonic schemes had forced rhythmic patterns to stay fairly uncomplicated. Around the turn of the century, however, harmonic and metric devices became either more rigid, or much more unpredictable, and each approach had a liberating effect on rhythm, which also affected ballet. Diaghilev was a pioneer in adapting these new musical styles to modern ballet. When Ravel used a 5/4 time in the final part of his ballet Daphnis and Chloé (1912), dancers of the Ballets Russes sang Ser-ge-dia-ghi-lev during rehearsals to keep the correct rhythm.

Members of Diaghilev's Ballets Russes later went on to found ballet traditions in the United States (George Balanchine) and England (Ninette de Valois and Marie Rambert). Ballet master Serge Lifar went on to revive the Paris Opera.

He died in Venice, Italy on August 19, 1929 and is buried on the nearby island of San Michele.