William Jay Sydeman (May 8, 1928 – May 27, 2021) was a prolific American composer. He was born in New York. He studied at Duke University, and received a B.S. degree in 1955 from the Mannes School of Music, having studied with Felix Salzer, Roy Travis, and Roger Sessions. He received his master's in music from the Hartt School in 1958, studying under Arnold Franchetti and Goffredo Petrassi. From 1959 to 1970 he joined the composition faculty at his alma mater Mannes School of Music.

In 1951 he married Hope Millholland, a pianist and fellow student at Mannes.

Winning early acclaim for his avant-garde music (principally published by C. F. Peters), he felt trapped by the prevailing orthodoxies and moved to California in 1970, beginning a period of wandering during which he also studied Buddhism and Anthroposophy. He joined ASCAP in 1975. From 1980 to 1982 he taught at Rudolph Steiner College in Fair Oaks. In 1981 he settled in Sacramento and resumed composition at his former prolific rate, having newly embraced a neotonal musical language. He later moved to Mendocino.

In 1966, his catalog (nearly 75 works by this date) was edited by Nancy B. Reich and published as an early exercise in machine-readable catalogs. A 2nd edition, published by NYU Department of Music Education, was released in 1968. 35 years later she would write his entry for Grove Dictionary of Music.

His music was commissioned from many leading institutions and performers. He won awards from the Boston Symphony Orchestra, the Pacifica Foundation, and the National Institutes of Arts and Letters.