Guillaume Costeley [pronounced Cotelay](1530, possibly 1531 – 28 January 1606) was a French composer of the Renaissance. He was the court organist to Charles IX of France and famous for his numerous chansons, which were representative of the late development of the form; his work in this regard was part of the early development of the style known as musique mesurée. He was also one of very few 16th century French composers of music for keyboard. In addition, he was a founding member of the Académie de Poésie et de Musique along with poet Jean-Antoine de Baïf, and he was one of the earliest composers to experiment with microtonal composition.