Léonin or ( also Léoninus, Léonius, Léo) was a French composer in the 12^th century. He was maître de chapelle (choir master) of the Church of Beatae Mariae Virginis (later Notre Dame of Paris). He is considered the first great representative of the Notre-Dame School and was teacher to Pérotin.

All that is known of Léonin is found in the writings of Anonymous IV, a later student at Notre Dame, who mentions him in a treatise on music theory. It is written that Léonin is known for being the first and best composer of polyphonic music. His organum or rather organum duplum, is arranged for two vocal lines of the Mass and the daily prayer hours. One vocal line is the chant tune, or cantus firmus, the other is a newly composed melodic descant.

The Magnus liber organi (the Great Book of Organum) is attributed to Léonin. It is a collection of two-voiced organum settings comprising nearly 100 Gregorian chants including Gradual, Alleluia, and Responsory chants. Léonin intended it to be sung for all the major feasts for the complete liturgical year. This repertoire remained in use for more than 2 centuries and spread to all the Western countries.