Harry Partch (June 24, 1901 – September 3, 1974) was an American composer, music theorist, and creator of musical instruments. He was one of the first 20th-century composers in the West to work systematically with microtonal scales. He built custom-made instruments in these tunings on which to play his compositions, and described his theory and practice in his book Genesis of a Music (1947 and 1974).

Partch composed using scales of unequal intervals in just intonation, derived from the natural Harmonic series; these scales allowed for more tones of smaller intervals than in the standard Western tuning, which uses twelve equal intervals to the octave. Partch divided the octave into 43 unequal tones. To play this music, he built a large number of unique instruments, with names such as the Chromelodeon, the Quadrangularis Reversum, and the Zymo-Xyl. Partch described his own music as corporeal, and distinguished it from abstract music, which he perceived as the dominant trend in Western music since the time of Bach. His earliest compositions were small-scale pieces to be intoned to instrumental backing; his later works were large-scale, integrated theater productions in which he expected each of the performers to sing, dance, speak, and play instruments. Ancient Greek theatre and Japanese Noh and kabuki heavily influenced his music theatre.

Encouraged by his mother, Partch learned several instruments at a young age. By fourteen, he was composing, and in particular took to setting dramatic situations. He dropped out of the University of Southern California's School of Music in 1922 over dissatisfaction with the quality of his teachers. He took to self-study in San Francisco's libraries, where he discovered Hermann von Helmholtz's Sensations of Tone, which convinced him to devote himself to music based on scales tuned in just intonation. In 1930, he destroyed all his previous compositions in a rejection of the European concert tradition. Partch frequently moved around the US. Early in his career, he was a transient worker, and sometimes a hobo; later he depended on grants, university appointments, and record sales to support himself. In 1970, supporters created the Harry Partch Foundation to administer Partch's music and instruments.