Leo Ornstein (December 2, 1892 - February 24, 2002) was an avant-garde composer and pianist.

Born in Kremenchug, Russia, Ornstein was one of the most popular radical composers in the United States, where he immigrated in 1907. A child prodigy, Ornstein mastered the piano at age eight. While he made no audio recordings, his playing was by all accounts world-class, and is preserved on numerous piano rolls he recorded for the Ampico label.

From the period 1910 to 1925, Ornstein was the giant of modern music being performed in the states. Among his most notable pieces I Allegro Barbero was one of the first fusion pieces, combining new musical techniques such as tone clusters and polyrhythms before they were widely used.

Ornstein gave countless performances in the 1910s and 20s, but in 1933, he retired from the stage because he disliked performing. Even though he had retired, Ornstein kept composing up until age 98.

In the mid-1970s, a renewed interest in his work began, and the question of his current whereabouts was raised. As it turns out, he was living in a mobile home in Texas, still writing music. He and his wife Pauline Mallet-Provost also founded the Ornstein School of Music in 1935 and operated it until it closed in 1958. In 1990, at the age of 98, Ornstein's final work, the Eighth Piano Sonata was completed and given its world premiere.

In early 2002, the radical composer died in a small nursing home in Green Bay, Wisconsin, at the age of 109.