Leo Ornstein (born Lev Ornshteyn; Russian: Лев Орнштейн; c. December 2, 1893 – February 24, 2002) was an American 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.Ornstein was the first important composer to make extensive use of the tone cluster. As a pianist, he was considered a world-class talent.