Guillaume de Machaut (French: [ɡijom də maʃo], Old French: [ɡiˈʎawmə də maˈtʃaw(θ)]; also Machau and Machault; c. 1300 – April 1377) was a French composer and poet who was the central figure of the ars nova style in late medieval music. His dominance of the genre is such that modern musicologists use his death to separate the ars nova from the subsequent ars subtilior movement. Regarded as the most significant French composer and poet of the 14th century, he is often seen as the century's leading European composer.

Machaut, one of the earliest European composers on whom considerable biographical information is available, has an unprecedented amount of surviving music, in part due to his own involvement in his manuscripts' creation and preservation. Machaut embodies the culmination of the poet-composer tradition stretching back to the traditions of troubadour and trouvère. His poetry was greatly admired and imitated by other poets, including Geoffrey Chaucer and Eustache Deschamps, well into the 15th century.

Machaut composed in a wide range of styles and forms and was crucial in developing the motet and secular song forms (particularly the lai and the formes fixes: rondeau, virelai and ballade). Among his only surviving sacred works, Messe de Nostre Dame, is the earliest known complete setting of the Ordinary of the Mass attributable to a single composer. Other notable works include the rondeaux "Ma fin est mon commencement" and "Rose, liz, printemps, verdure" as well as the virelai "Douce Dame Jolie".