Leonard Constant Lambert (23 August 1905 – 21 August 1951) was a British composer, conductor, and author. He was the founder and music director of the Royal Ballet, and (alongside Ninette de Valois and Frederick Ashton) he was a major figure in the establishment of the English ballet as a significant artistic movement.

His ballet commitments, including extensive conducting work throughout his life, restricted his compositional activities. However one work, The Rio Grande, for chorus, orchestra and piano soloist, achieved widespread popularity in the 1920s, and is still regularly performed today. His other work includes a jazz influenced Piano Concerto (1931), major ballet scores such as Horoscope (1937) and a full-scale choral masque Summer's Last Will and Testament (1936) that some consider his masterpiece.

Lambert had wide-ranging interests beyond music, as can be seen from his critical study Music Ho! (1934), which places music in the context of the other arts. His friends included John Maynard Keynes, Anthony Powell and the Sitwells. To Keynes, Lambert was perhaps the most brilliant man he had ever met; to de Valois he was the greatest ballet conductor and advisor his country had ever had; to the composer Denis ApIvor he was the most entertaining personality of the musical world.