Willem Breuker was a Dutch jazz bandleader, composer, arranger, saxophonist, and bass clarinetist.
In 1967, with percussionist Han Bennink and pianist Misha Mengelberg, he co-founded the Instant Composers Pool (ICP), with which he regularly performed until 1973. He was a member of the Globe Unity Orchestra and the Gunter Hampel Group.
In 1974 he began leading the 10-piece Willem Breuker Kollektief, which performed jazz in a theatrical and often unconventional manner, drawing elements from theater and vaudeville. With the group, he toured Western Europe, Russia, Australia, India, China, Japan, the United States, and Canada.