Leo Ornstein (born Лев Орнштейн, Lev Ornshteyn; c. December 11, 1895 – February 24, 2002) was an American experimental composer and pianist of the early twentieth century. His performances of works by avant-garde composers and his own innovative and even shocking pieces made him a cause célèbre on both sides of the Atlantic. The bulk of his experimental works were written for piano.

Ornstein was the first important composer to make extensive use of the tone cluster. As a pianist, he was considered a world-class talent. By the mid-1920s, he had walked away from his fame and soon disappeared from popular memory. Though he gave his last public concert before the age of forty, he continued writing music for another half-century and beyond. Largely forgotten for decades, he was rediscovered in the mid-1970s. Ornstein completed his eighth and final piano sonata in September 1990 at the age of ninety-four, making him the oldest published composer in history at the time (a mark since passed by Elliott Carter).