Jacob of Liège, aka Jacobus Leodiensis or Jacques de Liège (c. 1260-after 1330), was a Franco-Flemish music theorist, believed to have written the Speculum Musicae (The Mirror of Music) during the second quarter of the 14th century.
The Speculum Musicae, the largest surviving medieval work on music, was previously attributed to Jean de Muris by Edmond de Coussemaker. It now seems likely that it was written by someone with the name Jacobus (the initial letters of each of the seven chapters of the book spell out the acrostic IACOBUS). He was probably born in the diocese of Liège, before going to study in Paris in the late 13th century, and who returned to Liège to complete the final two books of his seven-volume treatise, Speculum musicae. Smits van Waesberghe associated him with Iacobus de Oudenaerde, professor at the University of Paris and canon of Liège, while he has also been identified with the Iacobus de Montibus mentioned in another manuscript.