Ernst Křenek (August 23, 1900 - December 22, 1991) was an Austrian-born composer; throughout his life he insisted that his name be written Krenek rather than Křenek, and that it should be pronounced as a German word.

He was born in Vienna. He studied there and in Berlin with Franz Schreker before working in a number of German opera houses as conductor. During World War I, Krenek was drafted into the Austrian Army, but he was stationed in Vienna, allowing him to go on with his musical studies. In 1922 he met Gustav Mahler's daughter, Anna, and her mother, Alma, who asked Krenek to complete her late husband's Symphony No. 10. Krenek helped edit the first and third movements but went no further. In 1924 he married Anna, only to divorce her before the first anniversary.

His music was banned in Germany by the Nazi Party and repressed in Austria. He moved to the United States of America in 1938 where he taught music, including at Hamline University in Saint Paul, Minnesota from 1942-1947. He became an American citizen in 1945. His students included George Perle and Robert Erickson. He died in Palm Springs, California.

Krenek's music is in a variety of styles. His early work is in a late-Romantic idiom, showing the influence of his teacher Schreker (de.) . He later embraced atonality, but a visit to Paris, where he became familiar with the work of Igor Stravinsky and Les Six led him to adopt neo-classical style. His opera Jonny spielt auf (Johnny Strikes Up, 1926), which is influenced by jazz, was a great success in his lifetime, playing all over Europe. He then started writing in a neo-Romantic style with Franz Schubert as a model, with his Reisebuch aus den österreichischen Alpen as prime example, before using Arnold Schoenberg's twelve tone technique; the opera Karl V (1938) is entirely written with this system, as are most of his later pieces. He also experimented with electronic and aleatoric music.

Krenek wrote a number of books, including Music Here and Now (1939) a study of Johannes Ockeghem (1953) and Horizons Circled: Reflections on my Music (1974).