Guillaume de Machaut (French: [gi'jom də ma'ʃo]; sometimes spelled Machault; c. 1300 – April 1377) was a medieval French poet and composer. He is one of the earliest composers on whom significant biographical information is available. According to Daniel Leech-Wilkinson, Machaut was "the last great poet who was also a composer". Well into the 15th century, Machaut's poetry was greatly admired and imitated by other poets, including Geoffrey Chaucer.
Machaut composed in a wide range of styles and forms. He is a part of the musical movement known as the ars nova. Machaut helped develop the motet and secular song forms (particularly the lai and the formes fixes: rondeau, virelai and ballade). Machaut wrote the Messe de Nostre Dame, the earliest known complete setting of the Ordinary of the Mass attributable to a single composer.